Tag Archives: Centrifugal Pumps

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.

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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.
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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.