Frequently Asked Questions

Our Frequently Asked Questions attempts to provide answers to the most commonly asked questions relating to our products and services. If you don’t find answers to your questions here, please get in touch with your nearest Stewarts & Lloyds branch. 

What is the difference between a centrifugal and positive displacement pump?

The characteristic of centrifugal pumps is the transmission of energy within the impeller blading on the basis of flow deflection; in contrast we have the volume displacement principle of positive displacement pumps, their total head is independent of the pump speed.

What type of pump is a peripheral pump?

Peripheral pump is a centrifugal pump with an impeller equipped with a large number of small radial vanes on either side of its periphery(approach flow from both sides of the impeller), their characteristic curves are steeper than those of regenerative pumps.

The shaft power of a peripheral pump decreases with increasing capacity.
Another feature of the peripheral pump is that it is able to pump relatively large amounts of gas with the fluid, in contrast to other centrifugal pumps,

Peripheral pumps are also able to continue operating under conditions of vapour bubble formation (cavitation) without any noticeable disturbance in their smooth running.

Disadvantages in comparison with conventional radial (impeller) pumps are the poor self-priming capability, noisy running and the low pump efficiency of peripheral pumps.

I keep hearing about suction recirculation but cannot find a good explanation. Can you help me?

When centrifugal pumps are operated at rates of flow below their best efficiency point (BEP) , the excess flow produced by the impeller is recirculated on the suction side of the impeller.

These eddy currents cause local vortices on the impeller vanes, which in turn cause cavitation resulting in noise , vibration, and damage to the impeller vanes.

This action usually begins between 70% and 50% of the BEP flow and many pump manufacturers do not allow operation below the onset of recirculation.

Should valves be open or closed when starting and/or stopping centrifugal pumps, what procedure is best to follow?

Pumps with a specific speed value below 6000, when primed and operated at full speed with discharge valve closed, require much less power input than when operated with the valve open.

For this reason, it is advantantageous to close or nearly close the discharge valve when the pump is  being started.

After start-up, prolonged operation at shut-off is harmful and should be avoided because of:

    Increased vibration levels affect the bearings and shaft seals
    Increased radial thrust and resultant stresses in the shaft and bearings
    Heat build-up resulting in dangerous temperature rise of the liquid being pumped
    Excessive cavitation damage resulting from internal recirculation

When shutting down, the discharge valve is usually closed first. If the discharge piping includes a check valve, no other valve closure is necessary.

What is the purpose of putting vacuum valves in pump discharge piping?

The primary purpose for vacuum breakers is to minimize the risk of damage from water hammer.

This is especially true when pumps are installed above an open source and discharging through pipe to a higher level. When the pump is stopped, the water in the discharge pipe may drain back to the source and cause contamination if a chemical was injected in the discharge pipe.

Even with a check valve in the discharge pipe, the check may leak and the water in the pipe can slowly drain out. When this happens, the falling water level in the discharge pipes creates a partial vacuum which  becomes filled with water vapour.

When the pump is restarted, it quickly fills the empty pipe with water until the water meets the first obstacle in the pipe system. At that time, the rush of water is abruptly stopped and the power of the rushing water is converted to pressure energy causing a high spike in pressure calling water hammer.

This pressure spike can and often does cause damage to the system. A longer discharge pipe will have a larger volume of moving water and result in a higher pressure spike.

If air is allowed into the pipe, it will cushion the flow of water and slow it down more gradually as the pressure builds. Water hammer and damage will be avoided. Vacuum release accomplish this.

The basic design includes a float device which is lowered when the water in the pipe is drained away, air is then allowed into the pipe and damage is avoided.

"Booster pumps" what are booster pumps and where are they used

Booster pumps are used to increase or “boost” liquid pressure in a system.Almost any pump might be called a booster pump.

The term is most frequently used to descripe a pump that is installed at the inlet to another pump to increase the liquid pressure and thereby increase the NPSH available to the other pump, which is frequently called the main pump.

The main pump could  be a centrifugal pump which requires more NPSH that is available from the system, a good  example is a high-pressure multistage pump.

The most critical requirement for a booster pump is low NPSH required, this is usually accomplished by operating the booster pump at lower speed.

My pump motor is exceeding full load amperage. What should I do?

There are several reasons why the pump may be exceeding full load amperage:

    Low voltage supply
    Three phase current is imbalanced
    Motor is grounded or shorted
    Wiring or connections are faulty
    Viscosity or specific gravity too great

The pump might also be undersized for the application and operating beyond it’s rated flow. In this case try throttling the discharge valve on the system back toward the closed position until pump  motor amps are within limits.

What is downthrust and the causes on a submersible borehole pump?

Downthrust is the download force that the impeller and shaft assembly experience when the pump is in operation.

Most pumps and motors are designed to operate under a continuous downthrust condition, but to much downthrust can create problems.

Downthrust problems occur when the pump is running at very low flows resulting in higher discharge pressures and higher downthrust loads.

Continuous operation in this range can damage the thrust bearing in the motor and may also cause overheating problems for the motor and pump due to lack of sufficient cooling flow.

To minimize downthrust problems, the pump must be run within its specified minimum and maximum flow range.

What is upthrust and the causes on a submersible borehole pump?

Upthrust is a condition where there is an upward force on the shaft impeller assembly.

This condition is caused when the pump is pumping more water than it was designed to produce ( pumping beyond the far right of the pump performance curve).

Upthrust does not typically occur in normal water well applications where a check valve is fitted on the pump outlet.

The static water column trapped in the pipe by the check valve creates a counter pressure to the pump and immediately loads the pump start-up, preventing operation operation at the extreme right side of the performance curve.

With no check valve, or with a leaky check valve, there is no fluid column to load the pump as it starts. With most pumps, this causes an uplifting on the impeller/shaft assembly in the pump. This upward movement carries across the pump/motor coupling and creates an upthrust condition in the motor.

While most motor manufactures have upthrust bearings which allow for limited upthrust without motor damage, it should be avoided to minimize wear in the pump and motor.

Repeated upthrust at each start can cause premature wear and failure of either or both pump and motor.

What is a cooling sleeve on a submersible borehole pump?

A cooling sleeve, also called a flow inducer sleeve, is a device used to make sure a submersible motor has enough water flowing over the surface of the motor to keep it cool.

In a water well installation the pump is surrounded by the well casing and the motor is mounted below the pump. As the pump draws in water from below, the water flows up the sleeve and over the surface of the motor and provides cooling for the water.

All borehole pumps installed in an open body of water required a cooling sleeve.

What are the most probable causes of insufficient discharge flow or pressure in centrifugal pumps?

We should look at pump system and check for the following:

    air leaks in the suction piping
    system head higher than anticipated
    insufficient NPSH available
    foot valve too small
    vortex at suction supply
    suction or discharge valve partially closed

Finally, looking at the pump:

    air leaks through the shaft seal or stuffing-box
    pump speed to low
    pump wearing rings worn
    damage to the impeller or internal blockage
    impellors loose on shaft (primarily on multistage)
    wrong direction of rotation

My end suction centrifugal pump takes suction from the bottom of an open tank, as the level in tank drops a vortex is formed which allows air into pump this changes the performance of the pump. Can I correct the problem by installing baffle plates?

Yes, and baffle plates are effective in correcting this problem.

The objective is to increase the apparent opening to the suction pipe, thereby reducing the liquid velocity as it leaves the tank. It is the velocity of the liquid flow that creates the vortex as the liquid level drops.

The simplest solution is the installation of a circular baffle plate with a diameter of four times the pipe diameter directly over the outlet pipe at a distance of two diameters above the pipe opening. This results in an apparent opening which is eight times the original opening and a corresponding reduction in flow velocity as the liquid moves under the baffle.

Of course the velocity increases under the baffle plate, but then it is protected from the surface air.

If the baffle plate is supported by four to six vertical plates forming a star to diameter of two times the pipe diameter, they will further reduce the start of a vortex.

Why is my electric motor running hot?

Because of modernized design and manufacturing techniques, it is typical for many of today’s motors to run hotter than those in the past.

In fact, it is not uncommon for the surface temperatures of some newer motors to reach as high as 90C while maintaining and even exceeding the lifetime of motors made in the past.

This means that a hot motor is not necessarily an indication that something is wrong. However, if the motor in your pump application seems running uncharacteristically hot, there are several situations that may causing this condition:


    Under or over voltage
    Unbalanced 3 phase power
    Loss of insulation resistance


    High ambient temperatures
    Lack of ventilation
    High altitudes (thin air)


    Excessive cycling
    To low or to high flow rate
    High rotating resistance by a damaged pump
    Obstruction in the pump
    High viscosity or specific gravity liquid