Pump Guy Article – September 2014 – Issue 33 – Assumed Knowledge

“Assumed Knowledge”

Hello Pump Guy

I work in maintenance at a rendering plant. We have many centrifugal pumps on site for moving industrial water and treated water. We have boilers, cookers and cooling towers. Every day we blast the walls and floors with high pressure soapy water.

We have no information on our pumps. The nameplates are illegible and scratched, or ripped off. We have no performance curves and we get no help from the local pump distributor, unless we want to purchase a new pump.

I always read the Pump Guy. You wrote earlier that the pump head is determined by the impeller diameter and speed. How can I use the impeller size and rpm to determine the head and probable performance curve? Any help you can offer is greatly appreciated.

Valiant S.

Hello Valiant,

I had the occasion recently to read the instructions on a bottle of shampoo. The instructions said, Lather! Rinse! Repeat! That’s all!

The instructions didn’t say my hair should be wet before applying the shampoo. The instructions didn’t say how much shampoo I should use. The instructions didn’t indicate how long I should leave the shampoo in my hair before rinsing, or how many times I should repeat the process. The shampoo manufacturer assumes too much regarding the users.

The pump industry also assumes too much regarding the users. There are certain things the pump industry knows but doesn’t often verbalize. So let’s verbalize the assumed rules of pumps.

There are three families of centrifugal pumps. The largest family is the classic centrifugal water pump because water is (and always has been) the most popular liquid transported in bulk with a pump.

The classic centrifugal water pump has a radial-flow impeller and generates all head by centrifugal action. This pump is excellent for pumping all liquids that flow approximately like water. This family represents about 75% of all centrifugal pumps.

Most centrifugal pumps are directly coupled to an electric motor. The most popular industrial electric motor is the 4-pole motor. With 50-Hz electricity, this motor rotates at 1,500-rpm. When coupled to a pump in a pipe system filled with liquid, the motor and pump rotate at about 1,450-rpm.

Before going further, let’s define some terms:

‘Head’ is a measure of energy. The units are ‘metres of head’. Head is converted into pressure for water by multiplying ‘metres of head’ by ten to get Kilopascals of pressure. We can also divide ‘metres of head’ by ten to get kg./sq.cm of pressure, or bars. If the liquid is not water, specific gravity (density relative to water) is factored into the conversion.

‘Shut-off Head’ (SOH) is the beginning of all pump curves. It represents maximum head at zero flow.

‘Best Efficiency’ is the optimum combination of head and flow through the pump at the least energy consumption.

‘BEP’ is Best Eff. Point, the head (m.) and flow (cu.m./hr) coordinates of best efficiency.

‘Best Efficiency Zone’ is approximately 90% to 110% BEP. Also called the preferred duty zone or the ‘sweet spot’ on the performance curve

The first rarely verbalized assumption is:

“At 1,500 rpm, the impeller diameter in mm, will yield a reliable approximate and repeatable ‘Shut-Off Head’ of the pump in metres. See the Graph.

Table 1

This means that a standard centrifugal pump with a 200-mm impeller on a 4-pole motor (50-Hz electricity) will generate approximately 13.5 m. of shut-off head (2nd column) and about 11-m of best eff. Head (3rd column). On interpreting numerous pump curves, you will see these numbers are accurate and repeatable within about 5%.

The second rarely verbalized assumption is:
The best efficiency point (BEP) for water pumps with radial flow impellers is about 85% of the shut off head. The actual BEP can vary from 82% to 88% of the shut-off head depending design specifics. You can see this on the graph (3rd column).

The third rarely verbalized assumption is:
The pump should be operated at, or close to the best efficiency point on the performance curve. Most pump companies don’t give instructions explaining what the operator or process engineer must do to control and hold the pump to Best Efficiency.
I liberally used the words ‘about’ and ‘approximately’ many times in the previous paragraphs. The unwritten assumptions are not precise like mathematics. The assumptions include variables depending on the pump design, the impeller design, impeller surface finish, blade count, liquid viscosity, internal clearances and the actual motor speed.

The assumptions don’t apply to pumps with mixed-flow, or axial-flow impellers, special duty pumps, or PD pumps. However, the assumptions cover about 3/4ths of all centrifugal pumps in a rendering plant.

If your motor speeds or impeller diameters are different, then we can apply the Affinity Laws. The Affinity Laws are covered in other Pump Guy articles in the 2KG Newsletters.

Now, let’s use this information and develop a probable performance curve for your pumps.

Typical Pump Curve

The graph shows the important elements of a typical centrifugal pump curve with a radial-flow impeller. The vertical axis reports head or elevation in metres starting at “0-m”. The vertical axis continues to Best Eff. Head and proceeds to “Shut-off Head” (SOH). Notice that best efficiency head is about 85% of the shut-off head. The horizontal axis reads flow in cubic metres/ hr. starting at “0″ rising through best efficiency flow and proceeds to maximum flow at zero head.

A centrifugal pump can push a liquid up into a vertical pipe, to a certain point beyond which no more elevation can be obtained. This is Shut-off Head. Flow is zero as all the energy of the electric motor and pump is invested into maintaining elevation.

Out on the curve is a point called BEP for Best Efficiency Point and coordinates drawn to best efficiency head in metres, and best efficiency flow in c.m./hr. In green, around the best efficiency head and flow is the preferred duty zone or ‘Sweet Spot’. This zone is where the pump is happy with minimal running vibrations, minimum heat generation and noise, and minimum component distortion.

Let’s say one of your pumps has a 225-mm impeller and is directly coupled to a 4-pole electric motor on 50-Hz electricity. The shut-off head (2nd column) would be approximately 17-m. The best efficiency head (3rd column) of this pump would be about 14-m. (85% of 17-m).

This pump would be good for a pipe system that contains about 14-m. of resistance. Or, this pump can generate about 140-kPa (4th column) of water pressure.

How do you know if a pump is performing as it should? In other Pump Guy articles, I discuss the importance of instrumentation.

Let’s say you have a water pump with a 250-mm impeller spinning on a 4-pole electric motor. The shut-off head would be about 21-m. Best efficiency head would be about 18-m. Ambient water at 18-m. would convert to about 180-kPa differential pressure.

With pressure gauges installed on the pump suction nozzle and discharge nozzle, open the valves, turn-on the pump, and allow the flow to stabilize. If the differential pressure across the pump is somewhere between about 170-kPa and 190-kPa, this pump is somewhere in the preferred duty zone, and close to best efficiency.

Who would have thought you could start with the speed and diameter of a pump impeller and develop a valid performance curve? You can use this information to develop your own performance curves.

It also shows how proper instrumentation contributes to pump reliability. Pumps need working gauges. Most pumps can be controlled to best efficiency by interpreting either differential pressure gauges or a flow meter. The pump is stressed when the differential pressure (or flow) varies from the projected readings.

Valiant, I hope this information helps. As you purchase new pumps, be sure to keep, and use an original performance curve. Nowadays, you can download many performance curves direct from your pump company website.

Stay in touch.