Vertical Centrifugal Multistage Pump Suction design - Eng-Tips

Hi All,

Seeking some advice, in order to increase/maximize my retention time of the tank(hold up volume),
I am designing the suction of a vertical multistage centrifugal pump(fr,ko,mn)(CRN), with an "inverted elbow suction" within the FRP tank.
This principle basically helps to bring down the minimum submergence.

eg. Total minimum submergence= 0.5 Diameter(from nozzle to bottom of the tank) + another 1.5D. At Point (1) in the diagram (theory taken off from sulzer hand book)

However, after a brief discussion with the vendor, as the vertical multistage pump's mech seal is at the upper side of the pump, vendor suggest that the minimum water level(Pump stop level) must still be at point(2)-above the mech seal to avoid air locking. is this correct?

Main question is whether the inverted nozzle design would be an advantage or end up making my minimum submergence worse than before.



Thank you!

Nick NPSHA is always calculated using absolute terms. It also must be given in terms of head of the liquid being pumped. The orientation of the nozzle doesn't effect the NPSHa of the pump except for the slight change in headloss of the elbow. Elbows that are downturned are used to allow the tank to be drained.

The pump manufacturer should be able to provide the NPSHr for the pump. Use the control system to ensure that the tank level doesn't go below that tank level.

If point #2 is the level that represents the NPSHr, then the water level should be keep above that level to avoid pump cavitation, not air locking.

Sometimes a vortex may occur on the intake and tank level should be kept above that point.

Thanks for the quick response!

Apologize for my poor drawing, this is a Grundfos CRN Pump, hopefully this image would be useful.
for NPSHr, it is about 4.24m, i believe it should not be a problem.

I am worried on the operating condition to stop the pump at minimum level.
Feed coming into this tank is inconsistent(depends on production) and therefore these pumps will be starting/stopping based on level.
And for this case i would like to stop the pumps when the level reaches (1) which is the most minimum level as per submergence calculations. instead of (2).




The CRN pump body have a vent hole that you can open and "prime" the pump with. As long as that vent stays shut and you don't drop the fluid level low enough to allow air into the suction of the pump the pump's case won't drain with the check valve in place. The pump needs to be fully vented of air and should be checked after a low level restart to make sure you didn't lose prime, but you'll only air lock the pump if you lose fluid and replace it with air. Is there some reason why there cannot be an isolation valve between the pump and tank ?

IMHO, this type of pump is a particularly BAD CHOICE when paired with a plastic tank. Is this a "repurposed" pump that you already have ?

If this is a new installation, why not buy a more conventional horizontal axis pump ?

Grundfos CRN multi-stage pumps have a very marginal base mounting and are prone to vibration as the pump gets older and worn ... The pump is basically an "out of control" cantilever ..... This will spell disaster for the FRP plastic tank nozzle.

How are you going to anchor this old used pump ? .... is the baseplate cracked or broken ? .... Send pictures

Additionally, you are confused and should not be talking about "total submergence" and "minimum submergence" ..... it is not appropriate for this type of pump. An evaluation of NPSH IS APPROPRISATE .... Do not confuse submergence with NPSH !!!

MJCronin
Sr. Process Engineer

Centrifugal pumps are an important part of current engineering applications. They do crucial work moving liquids from water supply systems to industrial processes efficiently. In the particular area of application; the global pump market, it is extremely important to grasp the ‘how’, to choose the best machine for our system, and to take best advantage of these investments through optimised performance. In this in-depth guide, we’ll cover everything you need to know about centrifugal pumps, including their types, features, design considerations, and even an easy-to-use pump calculation tool.

If you want to learn more, please visit our website.

What Is A Centrifugal Pump?

A centrifugal pump is a mechanical device designed to move fluids by converting rotational kinetic energy into hydrodynamic energy. It operates through the action of one or more driven rotors known as impellers, which increases the fluid’s velocity as it spins. Fluid enters the rotating impeller along it’s axis and is ejected by centrifugal force along it’s circumference through the impellers vane tips. This increased velocity is then converted into pressure, allowing the fluid to be transported through a piping system. Special design of the pump casing is such that the pump fluid is conveyed from pump inlet into impeller, then slowed and controlled before discharge.

These pumps are ideal for low-viscosity fluids such as water, fuel, or chemicals. However, their versatility makes them applicable in many industries, from agriculture to oil and gas.

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How Centrifugal Pumps Work

Centrifugal pump has an important component, known as the impeller. It is made from a series of curved vanes. They usually sandwich between two discs (an enclosed impeller). For slurry fluids an open or semi-open impeller backed by a single disc is preferred.

At the ‘eye’ of the impeller fluid goes in, and comes out around the vanes in the circumference. On the opposite side to the eye there is an impeller, which is connected to a motor and rotated at high speed (around 500-rpm) through a drive shaft. The fluid is thereby accelerated out through the impeller vanes into the pump casing at high rotational speed.

There are two basic designs of pump casing: volute and diffuser. We want a controlled discharge at pressure in both designs.

The impeller is situated in a volute casing, which acts to offset the impeller and in effect produce a curved funnel of increasing cross-sectional area to the pump outlet. This design leads to a pressure difference, increasing the pressure towards the outlet.

The same basic principle applies to diffuser designs. In this case, a set of stationary vanes surround the impeller with fluid being expelled between the two. Since diffuser designs can be tailored for particular applications, more efficient ones can be designed. When it is desirable to avoid the increased constrictions inherent in diffuser vanes, volute cases are more appropriate for applications with entrained solids or high viscosity fluids. Volutes of more than rectangular or square shape can cause the impeller to be worn and the drive shaft to wear more than equally.

Centrifugal pumps are constant head machines. They generate a fixed “head” (the height a fluid can be lifted) regardless of the fluid being pumped. The design and operation of a centrifugal pump are based on the relationship between the system curve (which defines the required flow and pressure) and the pump’s performance curve (which shows how the pump performs under various conditions).

A centrifugal pump is a mechanical device that, by means of an impeller driven at high speed by a prime mover, presses on the fluid to cause it to enter the pump casing and so move a fluid under pressure. The rapidly rotating impeller welcomes fluid along the axis and casts it out by centrifugal force along the circumference of the tip of the vane. The impeller action increases the fluid velocity and pressure as well as directs the fluid towards the pump outlet. The pump casing is specially configured to limit the fluid coming into the pump inlet, drive it into the impeller, and restrict on slowing and controlling the fluid before discharge.

Types of Centrifugal Pumps

Key Operational Characteristics of Centrifugal Pumps

Flow and Head Relationship

Unlike many other components, centrifugal pumps are designed for a specific combination of flow and head. Pumps run outside of their optimal design point (often known as their ‘best efficiency point’ or BEP’) decrease in efficiency and result in more component wear. This can be seen in Figure 3.

System Dependency

The system dictates the operating point of the pump, not the pump itself. If flow or pressure is not where it should be, either due to system or pump design, then it is not the pump but a problem with the system configuration or design which follows the system curve.

Efficiency Concerns

Pumps running away from their design point (e.g., higher flows or pressures than expected) will consume more power and exhibit lower efficiency. This can lead to early pump failure, cavitation, or even overheating.

Suction Conditions

Centrifugal pumps don’t “suck” fluid into the impeller. Instead, atmospheric pressure forces liquid into the pump. This is why adequate suction head or flooded suction is necessary, especially for higher elevation installations.

Pump Calculations | Finding the Right Size and Flow

An efficient and proper performing pump requires the proper selection and sizing of pump. According to the following formula, the essential parameters related to the centrifugal pump system can also be determined.

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Pump Head Calculation

The total dynamic head (TDH) for a pump system is the total height (in meters or feet) that a pump needs to lift the liquid, including friction losses.

The general formula for calculating the head is:

Where:

• Pd – Discharge Pressure (Pa or Psi)
• Ps – Suction Pressure (Pa or Psi)
• λ – Specific weight of the fluid (N/m3or lb/ft3)
• g – Gravitational acceleration (9.81 m/s2)
• Zd – Height at discharge (m)
• Zs – Height at suction (m)

Pump Head Calculator

This can be simplified to:

Where:

• Static Head is the vertical distance between the suction tank and the discharge point. (m)
• Friction Loss is the loss due to friction in the pipes and fittings. (m)
• Pressure Head is the pressure required at the discharge point. (m)

Total Dynamic Head Calculator

Flow Rate Calculation

To determine the flow rate needed for your application, use the following equation:

Where:

• Area is the cross-sectional area of the pipe (m2).
• Velocity is the speed of the fluid in the pipe (m/s).

It is important to note, that in many applications there is a range of velocity that is required for Certain pumping systems and piping networks. This is to reduce the chances of cavitation and also allow the liquid to move with enough velocity to reduce chances of settling if the fluid contains any solids.

Power Calculation

To calculate the power required for the pump, use this formula:

Where:

• Flow is in cubic meters per second (m³/s).
• Head is Height (m).
• Density is the fluid density in (kg/m³).
• Gravity is the acceleration due to gravity (9.81 m/s²).
• Efficiency is the pump efficiency (decimal).

Common Challenges in Centrifugal Pump Operation

1. Cavitation

Cavitation occurs when the pressure at the impeller’s inlet falls below the vapour pressure of the fluid. These bubbles, also called voids, collapse and cause shock waves that can damage the impeller, and reduce pump efficiency. Cavitation is a common cause of wear and tear in engineering, especially in pumps and propellers. 

2. Viscosity Handling

Centrifugal pumps are most effective with low-viscosity fluids. When dealing with higher viscosity fluids, pump performance decreases significantly, requiring adjustments to the design or a different pump type altogether.

3. Suction Lift Limitations

Most centrifugal pumps have limited suction lift capabilities. In cases where the fluid must be lifted from a lower level, a positive pressure or a flooded suction is required to maintain optimal operation.

Industrial systems can’t go without centrifugal pumps due to their simplicity, reliability and versatility. But choosing the pump that is right for your application requires understanding how all of that interplays: flow, head, system curves, and pump performance. By using the principles and formulas in this guide; you can optimise your pump system for efficiency and pump life.

Centrifugal pumps are fascinating devices for engineers and system designers due to their mix of design flexibility and operational stability. We hope this guide has helped you with knowledge and tools to make informed decisions when you’re considering your next centrifugal pump project.