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Net Positive Suction Head Cavitation in Centrifugal Pumps

How to Understand Net Positive Suction Head

There are two ways of expressing NPSH relative to a centrifugal pumping system.

Excerpt from the August 2020 Pumps & Systems article by Gary Dyson

To make the term net positive suction head (NPSH) more accessible to pump engineers who may not understand how to design an impeller for NPSH or the exact details of the physics, I have tried to simplify it:

  • NPSH is a measure of the absolute pressure energy present in a liquid. Pump engineers use this energy to help “feed” the fluid into the eye of the first-stage impeller. Pumps generally do not suck.
  • NPSH is the sum of the total static plus kinetic pressure minus the liquid vapor pressure at the pump suction nozzle or impeller entry, which is expressed in terms of head.
Net Positive Suction Head Cavitation in Centrifugal Pumps
Net Positive Suction Head Cavitation in Centrifugal Pumps

There are two ways of expressing NPSH relative to a centrifugal pumping system:

  1. NPSHa—The net positive suction head available is the measurement of the amount of fluid pressure energy available from the system at the pump impeller inlet.
  2. NPSHr—The net positive suction head required is the measurement of the amount of fluid pressure energy required by the pump.

The NPSH available to the pump should be more than what the pump requires. If there is not enough NPSHa, the pump will cavitate. As a result, the performance and reliability can be compromised.

Impeller Blade Cavitation from Bubbles
Impeller Blade Cavitation Damage from Collapsing Bubbles

NPSHr Curves

NPSHr curves, as provided by the pump manufacturer, are generated using the data collected during a pump performance test. Determining NPSHr requires testing the pump over a series of carefully controlled, constant flow points and varying suction conditions in a test facility. The test lab operator sets the flow and then begins to introduce a vacuum on the suction side of the pump. This reduces the suction pressure in controlled increments. While reducing the suction pressure, the discharge pressure is closely monitored.

>>Read more.

 


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Horizontal Centrifugal Pump Selection for Chemical Applications

Selecting the Right Pump for Chemical Applications

Review pump types and pick proper construction materials.

Excerpt from the February 2020 Pumps & Systems article by Pete Scantlebury

There are many considerations when selecting the right pump for chemical applications. When working with materials such as corrosive or flammable chemicals, extra care should be taken to ensure that the product selected is suitable.

Define the Fluid Characteristics

The safety data sheet (SDS) and chemical manufacturer or chemical distributor can provide required information such as fluid name, concentration, fluid temperature, specific gravity and viscosity at pumping temperature. If solids are present, determine at what concentration, particle size and hardness, and if the material is corrosive, flammable or combustible.

Describe the Application

The more detailed the description, the better. Make sure to verify:

  • What type of container is the chemical stored in? Is it a drum or tote, bulk storage tank, rail car or tanker truck?
  • Where will the fluid be moved? From a drum to a bucket, from a rail car to bulk storage, or simply recirculating in the same container, for example.
  • Is the liquid below the pump? In this case, a pump that is either self-priming or one that can be submerged in the liquid is needed.
  • What is the flow rate required? Flow rate is required to calculate friction loss in the piping system.
  • What is the total head or pressure required? Total head is based on the piping system and is used (along with flow rate) to help choose a pump.
  • What is the net positive suction head available (NPSHa)? It is the suction head made available to the pump and provided by the piping system. To avoid cavitation (causes erosion damage to pump components) the NPSHa must exceed the NPSH required (NPSHr).
  • How long will the pump be operating per day or week? This is important in evaluating energy costs. For example, if a pump is going to be operating many hours per day, a pump driven with an electric motor could have considerably lower operating costs compared to an air-driven pump.
  • Is it indoors or outdoors? What are the maximum and minimum ambient temperatures? This is important for the correct selection of the construction materials for the pump and motor.
  • What is the altitude? Higher altitudes reduce available lift if it is a self-priming application, reduces NPSHa and reduces cooling by an electric motors fan.

Pump Selection

There are many variables to the selection of the best pump type with the correct materials of construction.

  • Gather information on the fluid to be pumped.
  • Gather information on the hydraulic and application requirements.
  • Consult the experts. Consult with the chemical manufacturer, chemical distributor, pump manufacturers, local pump distributors, and even industrial supply catalogs that are experienced in the selection of chemical pumps.
Horizontal Centrifugal Pump Selection for Chemical Applications
Horizontal Centrifugal Pump Selection for Chemical Applications

Review Possible Pump Types

Here are some of the most common pump types for transferring chemicals.

Drum/barrel pumps: Ideal for transferring a wide variety of chemicals from pails, drums, totes [(such as intermediate bulk containers (IBCs)] and other containers used by chemical manufacturers to transport products to the user. These can be powered by electric, lithium ion battery or air motors.

Centrifugal pumpsProvide smooth flow and have a wide range of flows and head capabilities. These pumps are available in a wide range of materials and either mechanically sealed or sealless magnetically coupled. They are typically operated with electric motors.

Air-operated double diaphragm pumps: Versatile, simple to operate, and can pump solids and viscous fluids. They typically operate with compressed air.

Positive displacement pumps: Includes gear, rotary vane or piston pumps. These pumps are good with high viscosity fluids and can generate high pressures.

>>Read more.

 


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

A Step-by-Step Approach to Pump Selection

Consider pipe sizing and motor power requirements before picking your pump.

Excerpt from the September 2018 Pumps & Systems article by Lev Nelik

Unless a brand new plant is being designed, users decide to replace a pump because of its age and wear or persistent reliability issues.

Plant engineers typically spend their time with the process to make sure machinery is working, water is flowing, power is produced, lights are up and no environmental problems are developing. They are not, as a rule, experts on any particular type of machinery.

They are basically generalists, having learned to rely on qualified suppliers, who are experts within their particular niche (pumps, centrifuges, boilers, generators, etc.). When a pump fails, it is usually replaced with a new one, without much analysis or discussion. If it continues to fail frequently, a new supplier is approached for a better, more reliable pump.

Occasionally, a relatively minor modification to the process, like an addition of a cooling (or heating) piping loop, for example, is needed. It may not be a particularly complex system, and hiring major design contractors may not be economical for such a small project. Yet, it might still be beyond the expertise of the plant engineers, maintenance and operating personnel.

So, how is a pumping system, simple or complex, actually designed? Details of pump performance curves, types, pressure, power or efficiency are usually not on the horizon at this initial stage.

All the plant knows is their requirements. Maybe they want to pump 1,000 gallons per minute (gpm) from a cold water tank 2 miles away to a heat exchanger and return the water to the tank. Thus, the details of the pump will start to emerge.

1. Before talking about a pump, consider the pipe.

Velocity of liquid in pipes ranges between 3 to 10 feet per second (ft/sec). If the velocity is too slow, the dirt, sludge or other contaminants can settle. If flow is too fast, abrasive wear will reduce the life of the pipe. Plant designers are familiar with the specific concerns for each application. A sludge stream will have a larger pipe than a clean water application. But for a “nonexpert,” a good starting point could be, say, 5 ft/sec. Solving for pipe diameter (1,000 gpm, 5 ft/sec), we get d = 9.1 inches, so we round it to 10 inches to fit available pipe sizes. For now, we will not consider pipe schedule, wall thickness, etc.

2. Now that we have the pipe, pressure is the next step.

Pressure comes from friction and elevation. We will assume no elevation changes along the pipe run. Friction losses are determined from a well-known Moody Diagram, from which a friction coefficient is found and then friction losses (h) are calculated (see Image 1).

This is the friction loss a pump pressure would need to work against.

The Moody Diagram has lots of helpful information on it: Reynolds number (Re), pipe type/age, roughness, and thus friction coefficient, as seen on Image 1, may range from 0.01 to 0.1, potentially an error. Fortunately, some of this can be simplified.

Re = 5 ft/sec x (10/12) (ft) / 10-6 = 4 x 106 – i.e. turbulent region and, from Image 1, we already cut down the friction factor to start from at least 0.2. If we reduce this region from the rough pipe and super smooth pipes, we find that an average will be around f = 0.03 for an iron pipe of 10 inches diameter.


>>Read more.

 


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Horizontal End Suction Pumps

Back to Basics: Pump Types – End Suction Vs. Double Suction

There are often multiple pump types (Table 1) that can be selected for the same water application, with each pump type having its own strengths and weaknesses. This column tries to help guide the reader in the selection of the best pump type that will yield the greatest reliability and lowest life cycle cost for a specific application.

Excerpt from the WaterWorld August 2011 issue by Allan R. Burdis

End Suction Water Pumps

An end suction water pump would probably have the lowest initial cost for most applications, with reasonable efficiency. However, these pumps do not follow any standards, especially with regard to bearing life, shaft seal housings and dimensional interchangeability. They are also typically constructed with the lowest cost materials, such as cast iron casings with bronze or brass impellers. The impellers are typically of closed construction, without replaceable casing or impeller wearing rings. Further, there is typically more deviation from published performance, such as efficiency, for this pump type.

Rotary Pump Power

For non-critical, intermittent service applications these pumps may be the best choice. However, for critical applications, requiring long operating life, the cost of maintenance and down time may far exceed any initial cost savings.

 

End Suction ANSI/ASME B-73 Pumps

Chemical pumps (figure 1), which can handle corrosive, and/or toxic liquids and slurries, are available in a variety of configurations and materials. Pumps used in this industry are different from those used in other industries, primarily in the materials of construction and the many mechanical shaft seal configurations available. These pumps must also meet the American Society of Mechanical Engineers ANSI B73 standards, which require dimensional interchangeability, minimum bearing life, and many other quality specifications. The minimum casing material is ductile iron, with stainless steel being quite common. Typical construction is an adjustable open impeller, which is also good at handling entrained air.

Because of these upgraded features, reliability focused users will typically select an ANSI/ASME B-73 pump over a lower cost water pump for other critical applications, including water services.

Read More


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Pump Specific Speed

5 Must-Read Articles About Pump Efficiency

Pump efficiency is a fundamental concern for every end user. A thorough understanding of the definition of efficiency and what factors affect it can help operators and engineers optimize equipment design and operation. The five articles listed below are some of Pumps & Systems most well-read resources that outline efficiency concepts every end user should know.

Excerpts from Pumps & Systems

1. Centrifugal Pump Efficiency—What Is Efficiency?

Efficiency is simply how well a machine can convert one form of energy to another. If one unit of energy is supplied to a machine and its output, in the same units of measure, is one-half unit, its efficiency is 50 percent. As simple as this may seem, it can still get a bit complex because the units used by our English system of measurement can be quite different for each form of energy. Fortunately, the use of constants brings equivalency to these otherwise diverse quantities.
>>Read more.

2. Centrifugal Pump Efficiency—Specific Speed

As Terry Henshaw stated in “Centrifugal Pump Specific Speed”, the definition of specific speed can be confusing. It is best to think of it as an index number that can predict certain pump characteristics. Viewed this way, specific speed can be useful when selecting a pump for a particular application and predicting premature failure due to off best efficiency point (BEP) operation.
>>Read more.

3. The Power of Wear Rings Part Two: Efficiency

For decades, pump designers have known that increasing wear ring clearance leads to a loss of efficiency. However, with metal wear rings, even the minimum clearance as specified by API610 is substantial. Because the clearance cannot be reduced between two metal rings without an increased risk of pump seizure, metal wear rings limit pump efficiency.
>>Read more.

efficiency gain

4. Pumps: HP, RPM and Energy Efficiency

Reduced operating speed translates into less bearing wear and longer motor life. Longer pump seal life is achieved and the damaging effects of abrasives in the recirculated water are reduced. Less wear and longer life means a reduction in maintenance costs and system downtime.
>>Read more.

5. Gain Efficiency with Volute Design

The volute serves a simple purpose in radial machinery: to transfer flow from an annular cross section to an exit pipe. Of course, there are many variations on the theme. The process is reversed for turbines, where more than one exit or inlet to the volute may exist, and many other arrangements are possible.
>>Read more.


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Centrifugal Pump Discharge Nozzle

How Centrifugal Pumps Work

How Does a Centrifugal Pump Work?
Excerpt from Pumps & Systems, May 2018, by Hydraulic Institute

The design behind this common pump type.

A centrifugal pump is a type of rotodynamic pump that uses bladed impellers with essentially a radial outlet to transfer rotational mechanical energy to the fluid primarily by increasing the fluid kinetic energy (angular momentum) and increasing potential energy (static pressure). Kinetic energy is then converted into usable pressure energy in the discharge collector.

Centrifugal Pump Discharge Nozzle
Centrifugal Pump Discharge Nozzle

A cross section view of a centrifugal pump would show the use of a rotating impeller to add energy to the pumped liquid. The liquid enters the impeller axially at a smaller diameter, called the impeller eye, and progresses radially between the vanes until it exits at the outside diameter. As the liquid leaves the impeller, it is collected in a pressure container casing. One design, referred to as a volute, collects the flow and efficiently directs it to a discharge nozzle. Image 1 highlights the discharge nozzle, which is shaped like a cone so that the high-velocity flow from the impeller is gradually reduced. This cone-shaped discharge nozzle is also called a diffuser. During the reduction in velocity in the diffuser, energy in the flow is converted to pressure energy. An optimum angle of seven to 10 degrees is used to most efficiently convert velocity energy to pressure energy.

Centrifugal pumps can have many drivers, but the most common is the electric motor. The motor provides the mechanical energy to pump shaft through a coupling. The radial and axial loads are carried by pump and/or motor bearings. Sealing the pumped fluid can be done with compression packing or mechanical seals. Additionally, sealless designs are available with canned motors or magnetic drive couplings.

Read more at Pumps & Systems.


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Horizontal Centrifugal Pump

Characteristics of Centrifugal Pumps

What are the Characteristics of a Centrifugal Pump? Head, performance curve and affinity laws all contribute to the efficiency of centrifugal pumps.

Excerpt from Pumps & Systems, Sept. 2012, by Sharon James, Rockwell Automation

Pumps are generally grouped into two broad categories—positive displacement pumps and dynamic (centrifugal) pumps. Positive displacement pumps use a mechanical means to vary the size of (or move) the fluid chamber to cause the fluid to flow. On the other hand, centrifugal pumps impart momentum to the fluid by rotating impellers that are immersed in the fluid. The momentum produces an increase in pressure or flow at the pump outlet.

Horizontal Centrifugal Pump
Horizontal Centrifugal Pump

Positive displacement pumps have a constant torque characteristic, whereas centrifugal pumps demonstrate variable torque characteristics. This article will discuss only centrifugal pumps.

A centrifugal pump converts driver energy to kinetic energy in a liquid by accelerating the fluid to the outer rim of an impeller. The amount of energy given to the liquid corresponds to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller, then the higher the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.

Characteristics

Creating a resistance to the flow controls the kinetic energy of a liquid coming out of an impeller. The first resistance is created by the pump volute (casing), which catches the liquid and slows it down. When the liquid slows down in the pump casing, some of the kinetic energy is converted to pressure energy. It is the resistance to the pump’s flow that is read on a pressure gauge attached to the discharge line. A pump does not create pressure, it only creates flow. Pressure is a measurement of the resistance to flow.

Static Discharge Head Suction Lift and Total Static Head

Figure 2. Representation of static discharge head, static suction lift and total static head

Head—Resistance to Flow

In Newtonian (true) fluids (non-viscous liquids, such as water or gasoline), the term head is the measurement of the kinetic energy that a centrifugal pump creates. Imagine a pipe shooting a jet of water straight into the air. The height that the water reaches is the head. Head measures the height of a liquid column, which the pump could create resulting from the kinetic energy the centrifugal pump gives to the liquid. The main reason for using head instead of pressure to measure a centrifugal pump’s energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. End users can always describe a pump’s performance on any Newtonian fluid, whether it is heavy (sulfuric acid) or light (gasoline), by using head. Head is related to the velocity that the liquid gains when going through the pump.

All the forms of energy involved in a liquid flow system can be expressed in terms of feet of liquid. The total of these heads determines the total system head or the work that a pump must perform in the system. The different types of head—friction, velocity and pressure—are defined in this section.

Friction Head (hf)

Friction head is the head required to overcome the resistance to flow in the pipe and fittings. It depends on the size, condition and type of pipe; the number and type of pipe fittings; flow rate; and nature of the liquid.

Velocity Head (hv)

Velocity head is the energy of a liquid as a result of its motion at some velocity (V). It is the equivalent head in feet through which the water would have to fall to acquire the same velocity or, in other words, the head necessary to accelerate the water. Velocity head can be calculated using the following formula:

hv = V2/2g

Where:
g = 32.2 ft./sec.2
V = liquid velocity in ft./sec.

The velocity head is usually insignificant and can be ignored in most high-head systems. However, it can be a large factor and must be considered in low-head systems.

Pressure Head
Pressure head must be considered when a pumping system either begins from or empties into a tank that is under some pressure other than atmospheric. The pressure in such a tank must first be converted to feet of liquid. A vacuum in the suction tank or a positive pressure in the discharge tank must be added to the system head, whereas a positive pressure in the suction tank or vacuum in the discharge tank would be subtracted. The following is a formula for converting inches of mercury vacuum into feet of liquid:

Vacuum, feet of liquid = (Vacuum, in. of hg x 1.13)/Specific Gravity

The different types of head are combined to make up the total system head at any particular flow rate. The descriptions in this section are of these combined or dynamic heads, as they apply to the centrifugal pump.

Read more at Pumps & Systems.


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Horizontal Centrifugal Pump

Pumping Solids In Chemical Processing Applications

Centrifugal Pumps for Pumping Solids In Chemical Processing Applications

Excerpt from Process Industry Informer, Oct. 15, 2015, by Malcolm Walker

Pumps designed to handle solids are often not the cheapest option. However, when you consider the costs of maintenance, production downtime, or even cleaning up escaped corrosive liquids, they often represent the cost-effective option for many applications.

Consider the centrifugal pump when selecting pumps for handling solids. There is a misconception that there aren’t any chemically-resistant centrifugal pumps capable of handling solids. It is a situation that needs correcting.

Horizontal Centrifugal Pump
Horizontal Centrifugal Pump

Centrifugal pumps featuring a stainless steel vortex impeller provide an excellent and well-proven solution. In operation, a whirlpool effect is formed in the volute by the rotating recessed cup-shaped impeller extending into the suction line, drawing the liquid/solids into the pump and then quickly through the discharge.

This motion minimizes the fluid/solids contact with the impeller and volute. It is a simple design that allows solids to pass through the pump without choking the impeller, so considerably reducing pump wear and the potential for blockages.

The fully recessed cup-shaped impeller and free passageways within the pump casing offer almost total freedom from blockages where solids and fibrous materials can be encountered in process waste water treatment plants.

Read more at Process Industry Informer.


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Horizontal End Suction Pump

Article Review: 4 Factors to Consider for Irrigation Pumps

Determine how booster and suction lift applications are used.

Originally published in the February 2019 issue of Pumps & Systems by Justin McDaniel

Irrigation pump requirements are influenced by the type and scope of the system to be installed. Gathering some basic information from the irrigation plan is essential when selecting the correct pump for the application.

Specific site requirements may also influence pump selection. If the site has a limited window when watering can occur, the pumping system must be sized to deliver all of the water required during that window. An undersized pump prevents the system from meeting the total irrigation requirements during the hottest periods.

Horizontal Centrifugal Pump
Horizontal Centrifugal Pump

Additionally, the type of sprinkler selected and the location for the pump can have a material impact on the cost of the pumping equipment and long-term system operating costs. In general, pump selection is governed by four factors:

1 Required operating pressure. Gather the operating pressure requirements from the irrigation plan, noting the required maximum pressure.

2 System flow. It is important to understand the scope of the system.

3 Friction losses. Calculate the total head loss and elevation change from the outlet of the pump to the highest sprinkler head, as well as the suction lift head requirements.

4 Budget. The design of an irrigation system, as well as the site characteristics, can have dramatic ramifications for the cost of the pump. The available power supply is always an important factor to consider and may also impact the cost of the pump.

Common Irrigation Pumps
There are two main actions that a pump performs in an irrigation application: pressure boost or suction lift. The four styles of pumps that are typically used for these applications:

  1. horizontal centrifugal
  2. submersible
  3. vertical turbines
  4. vertical multistage pumps

Boost Applications
Booster pumps are used to increase (boost) the water pressure in the system. Primarily, booster pumps are used on domestic or municipal water systems when the existing pressure is not adequate for the irrigation system.

The two most widely used are horizontal centrifugal and vertical multistage. The selection of the pump depends on three factors:

  1. pressure is required to run the system
  2. what type of efficiency must be reached
  3. the type of water: municipal, reclaimed water or gravity supply tanks

Vertical multistage pumps are designed to handle clean water that is free from large solids and debris. Vertical multistage pumps lend themselves to higher pressures, whereas horizontal centrifugal pumps lend themselves to a lower maximum pressure range but are generally more forgiving when handling dirty water.

Horizontal centrifugal pumps and vertical turbines can handle dirty water as long as a screen is present. The screen is important because, without it, the irrigation system can get plugged up. Submersibles and vertical multistage pumps are recommended for boosting clean water. Solids or debris can damage these without the use of components such as filters.

Suction Lift Applications
In a suction lift application, the system does not have incoming pressure but is instead pulling from a water source, such as a lake, river, sump, etc. Suction lift applications commonly require the use of horizontal centrifugal pumps and vertical turbines. An advantage of horizontal centrifugal pumps is that they are easier to install and maintain because the internal parts are easily accessible. Vertical turbines and submersibles are suited to suction lift applications since they are located in the water and do not need to create the vacuum to lift water to the pump intake.

Variable Frequency Drives & Irrigation Systems
VFDs are not required for smaller irrigation systems with nonvarying flows. They are recommended for larger systems to enable soft starts and energy reduction.

Click the link for the full article in Pumps & Systems magazine February 2019.


Industry News from the Pumps & Systems


A Complete Line of Pumps for Industry

Vertiflo Pump Company’s Vertical Sump Centrifugal Pumps, Horizontal End Suction Centrifugal Pumps and self-priming pumps are delivered fast, usually in half the typical lead time. Vertiflo’s vertical sump pump line offers up to 3000 GPM, 250′ Heads and 26′ depth. The horizontal end suction pump line offers up to 3000 GPM and 300’ Heads.

Vertiflo pumps are designed for nonresidential applications and currently over 20,000 are operating successfully worldwide. Vertiflo is recognized as a quality manufacturer of dependable pumps, and continues to grow and encompass new applications in the pump industry.

Suction Throttling NPSH Test Setup

What You Need to Know About NPSH

Most pump problems are related to net positive suction head (NPSH).
By Terry Henshaw, Pumps & Systems February 2018

Definition of NPSH
The margin of pressure over vapor pressure, at the pump suction nozzle, is net positive suction head (NPSH). NPSH is the difference between suction pressure (stagnation) and vapor pressure. In equation form:

NPSH = Ps – Pvap

Where:
NPSH = NPSH available from the system, at the pump inlet, with the pump running
Ps = stagnation suction pressure, at the pump inlet, with the pump running
Pvap = vapor pressure of the pumpage at inlet temperature

Since vapor pressure is always expressed on the absolute scale, suction pressure must also be in absolute terms. In U.S. customary units, both pressures must be in pounds per square inch absolute (psia). Gauge pressure is converted to absolute pressure by adding atmospheric pressure. The above equation provides an answer in units of pressure (psi). This can be converted to units of head (feet) by the following equation:

h = 2.31p/SG

Where:
h = head in feet
p = pressure in psi
SG = specific gravity of the liquid

Importance of NPSH
NPSH is a subject of extreme importance in all pumping systems. It has been estimated that 80 percent of all pump problems are due to inadequate suction conditions, and most suction problems are related to NPSH. (Either the system does not provide as much as anticipated, or the pump requires more than anticipated.) It is therefore probable that most pump problems are NPSH problems.

Units of NPSH
For centrifugal pumps, NPSH values are expressed in units of specific energy (equivalent column height) such as feet or meters. For displacement pumps (rotary and reciprocating), NPSH values are normally expressed in pressure units such as pounds per square inch (psi), kilopascals pr bars.

NPSH values are neither gauge pressures nor absolute pressures. The “g” in psig means that the pressure is measured above atmospheric pressure. The “a” in psia means that the pressure is measured above absolute zero, a perfect vacuum. NPSH is a measurement of pressure above vapor pressure, so the units of NPSH (in the U.S.) are just psi or feet.

NPSH Available: A System Characteristic
NPSHa stands for NPSH available from the system. It can be calculated by measuring suction pressure at the pump suction nozzle, correcting to datum, adding atmospheric pressure, adding velocity head and subtracting vapor pressure. In equation form:

NPSH = Psg + Pz + Patm + Pvel – Pvap

Where:
NPSHa = NPSH available to the pump, psi
Psg = gauge pressure measured at suction nozzle, psig
Pz = elevation of gauge above pump centerline, converted to pressure unites, psi
Patm = atmospheric pressure, psia
Pvel = velocity head, converted to pressure united, psi
Pvap = vapor pressure of the pumpage, at the pump suction nozzle, psia

If desired, all units can be converted to head (feet) prior to plugging into the equation. If the system has not been built, it is necessary to calculate the NPSHa by starting with the pressure in the suction tank. Add atmospheric pressure, add (or subtract) the liquid level above (below) datum, subtract all losses from the tank to the pump and subtract vapor pressure.

NPSH Required: A Pump Characteristic
The letters NPSHr stand for the NPSH required by the pump. This characteristic must be determined by test.
For proper operation of the pump, it is necessary that NPSHa > NPSHr. The system must provide more NPSH than the pump requires.

What Happens When NPSHr Exceeds NPSHa?
One potential complication when NPSHr exceeds NPSHa is cavitation. If at any time the static pressure on a liquid drops below vapor pressure, a portion of the liquid will boil-it will flash to a gas. This formation of gas bubbles is called cavitation. (Cavities form in the liquid.) Such gas formation in a suction pipe or inside a pump may cause a reduction in pump capacity and/or head. It may also cause damage to the pump. As liquid flows into any pump, there is a reduction in pressure. In a centrifugal pump, the liquid accelerates into the eye of the impeller, causing a reduction in pressure. The impeller vanes then slice into the liquid, creating zones of lower pressure.

If a sufficient pressure margin, over vapor pressure, is not provided at the pump inlet, some of the liquid will flash at the leading edge of each vane.

With displacement pumps, the situation is similar. Because the pressure drops as the pumpage moves into the pumping chamber, suction pressure must exceed vapor pressure by some margin to prevent cavitation.

Even though the liquid is cavitating, we usually say that the pump is cavitation. Possible effects of pump cavitation include noise, head and/or capacity loss, and equipment damage. It is not the formation of the bubbles that causes damage. Damage is caused to pump parts when the bubbles collapse or “implode.” When the bubbles collapse on a hard surface, they create a high pressure.

To Quieten a Cavitating CentrifugaI Pump
If a centrifugal pump sounds as if it is pumping gravel, if the system can tolerate it and if no other solution is readily available, inject about ½ percent (by volume) air (or other gas) just upstream (into the inlet) of the pump. Experiment to see how little air is necessary to obtain quiet operation. (Do not try this with a reciprocating pump!)

NPSHr & Suction Specific Speed
Suction specific speed, like specific speed, is not a speed at all. It is an index number, or “yardstick.” It is based on the NPSHr of a centrifugal pump, normally the 3 percent head drop NPSHr and normally at its best efficiency point (BEP). The equation for suction specific speed is the same as specific speed, except that NPSHr is substituted or head, as follows:

S = N√Q/NPSHr0.75

Where (in U.S. units):
S = suction specific speed
N = RPM of pump
Q = pump capacity*†, GPM
NPSHr = NPSH required by pump†, feet

*If the impeller is double suction, Q in the above equation is one-half the BEP capacity of the pump. This is a major difference from calculating specific speed, in which w use total pump capacity, whether the impeller is single suction or double suction.
†Normally calculated at the BEP

The symbol Nss is often used in place of S for suction specific speed. The value of S for most pumps is typically between 7,000 and 15,000. The higher values are more common in higher speed, higher capacity units.

Suction Specific Speed
For a number of years, the push from users and competitors required pump manufacturers to continually strive for lower values of NPSHr. The philosophy was that “The lower the NPSHr, the better the pump.” NPSHr in centrifugal pumps is normally reduced by increasing the diameter of the impeller eye. That philosophy has now changed.

Due to problems that have been attributed to oversized impeller eyes, pump users have established maximum values for S, which establishes minimum values for NPSHr. (See Pumps & Systems, December 2011, pumpsandsystems.com/ analyzing-impeller-eye).

Every centrifugal pump would like to run at its BEP. Pump components would experience maximum life at that capacity. Seldom does a pump run at its BEP, but component life will be significantly extended if it operates within its “stable” window of capacities. Suction specific speed can indicate the size of that window. Pumps with lower values of S have larger windows.


WAYS TO INCREASE & REDUCE NPSHa
If it is necessary to increase a system’s NPSHa, one or more of the following steps may be employed:

  1. Raise the level of the liquid in the suction vessel.
  2. Cool the liquid (after it leaves the vessel).
  3. Reduce the losses in the suction line by reducing its length, increasing its diameter, reducing number of fittings, etc.
  4. Install a booster pump.
  5. With a reciprocating pump or pulsating rotary pump, install a bottle or suction stabilizer in the suction line adjacent to the pump.

Ways to Reduce NPSHr
If the pump is in the selection stage, one or more of the following options may be employed to reduce the NPSHr:

  1. Use a double-suction impeller.
  2. Use a larger pump.
  3. Use a lower speed pump.
  4. Use an inducer (a small axial-flow impeller built into the eye of the main impeller).
  5. Install the pump at a lower elevation.
  6. With a vertical turbine pump, lower the first-stage impeller (make the pump longer).

All of the above will normally increase the initial cost of the pump and/or installation. Options two and three will also likely result in higher operating cost due to lower efficiency. Option two may result in a pump operating in the hydraulically unstable range. If the pump is already installed, one or more of the following options may be employed to reduce the NPSHr:

  1. If operating near or beyond the SEP, reduce pump capacity.
  2. If operating near shut-off, increase pump capacity (with a bypass if necessary).
  3. Reduce wear ring clearances.
  4. Reduce seal flush flow.
  5. Vent the seal chamber (stuffing box) back to the suction vessel.
  6. On a horizontal multistage pump with a balancing drum, pipe balancing line back to the suction vessel. The valves in this line must be locked open.
  7. On a vertical multistage pump, pipe the bleed-off line from the throat bushing back to the suction vessel. The valves in this line should be locked open.

This article was originally printed in Pumps & Systems magazine February 2018.


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