Explore Pump Hydraulic Calculations, Operation, and Control

Pump should always be handling liquid only.

The least portion of vapor in the fluid can cause pump cavitation.

So to make sure cavitation shall not occur, an available Net Positive Suction Head (NPSH) should be calculated.

Now let’s dive into another important calculation that is performed when designing a pump. This is related to the differential head calculation.

Differential pressure sums all the static and dynamic pressure losses in the pump discharge in addition to the pressure that is targeted at the destination and then subtracted from suction pressure.

Notice we are still talking about pressure. However, the characteristic pump curve at a specific flow rate is related to the head not to the pressure, which means that it doesn’t depend on density.

So let’s see this example:

if the pump differential head at a specific flow rate is 100m, the differential pressure when pumping water shall be around 10 bar. However, if the pump is pumping a fluid with a specific gravity of 0.7 at the same volumetric flow rate, the pump differential pressure shall be only 7 bar. So this is what we mean by saying that the pump output would be head, not pressure.

Pump strainers are filters that are used upstream the pump to make sure the fluid is clean and has no solids. That’s why it is susceptible to plugging during start-up due to accumulation of construction debris in the pipe.

Pumping a liquid at bubble point from a vessel, how would this affect the pipe size, vessel elevation and strainer pressure drop?

Pumping condensate from a tank.

Now let’s dive into another important calculation that is performed when designing a pump. This is related to the differential head calculation.

Differential pressure sums all the static and dynamic pressure losses in the pump discharge in addition to the pressure that is targeted at the destination and then subtracted from suction pressure.

Notice we are still talking about pressure. However, the characteristic pump curve at a specific flow rate is related to the head not to the pressure, which means that it doesn’t depend on density.

So let’s see this example:

if the pump differential head at a specific flow rate is 100m, the differential pressure when pumping water shall be around 10 bar. However, if the pump is pumping a fluid with a specific gravity of 0.7 at the same volumetric flow rate, the pump differential pressure shall be only 7 bar. So this is what we mean by saying that the pump output would be head, not pressure.

After sizing the suction system of the pump pumping bubbled fluid, let's see the discharge system, how would flow elements, pipe resistance and exchanger pressure drops affect the pump. How would the tower static head affect the required pump differential pressure?

Here we mainly have a pipeline, how would we optimize the pipe size against the pipe rating. Remember, a small discharge pipe size means more pump differential pressure, this means that the pipe should withstand a higher design pressure. This means a higher piping class, flange rating, and thickness. How can we optimize the system?

Pump power is important to size the pump motor and to take into consideration in plant power generation system. Power depends on pump efficiency specified by pump supplier as well as other factors.

So this is an example of a performance curve for a centrifugal pump provided by the pump manufacturer. It represents the flow versus head curve.

We plot the pump performance curve against the discharge dynamic and static pressure to get the pump operating point. Let's see how.

Let’s go deeper in the pump curves and explore their important aspects.

Pump manufacturers usually provide the curves based on more than one impeller, just as a case for the future if the conditions changed and we needed a larger or smaller impeller. What is the pump shutoff pressure? End of curve point? BEP? Let's see.

We have shown a typical curve for a centrifugal pump, but what about reciprocating or positive displacement pumps? Let's see how their curve works

Shall we add a control valve on the discharge or use a variable frequency drive motor?

Shall we need a minimum flow or recycle line with a flow control or a restriction orifice? How would this affect pump operation and efficiency?

The pump head is proportional to the root of the pump speed. And pump power is proportional to the pump head. How would this save the pump power consumption?

The most common pump configuration is to use a single pump with a spare pump.

However, in some cases we may need to use more than one pump in operation. This can be using pumps in series or in parallel. Here we shall discuss how each configuration works, its advantages and disadvantages, and when we use it.

How to operate two pumps in parallel? How can we balance between both pumps? Do the pumps need to be identical? Should their piping be symmetrical?

When do we use a booster pump before the main pump? How is this related to pump NPSH?

The below is a part of  "Control Valve Hydraulics"  course. It is showing how the pump curve and control valve characteristics are affected by the pressure drop in a pipe. If you'd like to know more about pump and control valve hydraulics, you can check out the course with an exclusive discount in the bonus lecture.

The control valve inherent characteristics provided by the valve manufacturer are relative to the valve only.

But in the actual case, the valve is not alone, there is a system upstream the valve and a system downstream the valve.

So here comes an important concept which is the control valve authority. This represents the fraction of control valve pressure drop to the total system pressure drop.

We can see it in the this equation.

So when talking about the inherent valve characteristics, it is considering the whole pressure drop is only in the valve, which means that the valve authoity is 1.

But when we put the valve in the system, we have lower pressure at the valve inlet, this is because of there is a pressure drop in the upstream piping or equipment.

Also because there is a pressure drop downstream the valve, this means that there will be a higher backpressure on the valve.

So we will have less pressure drop to be exerted by the valve and depending on the flow rate, the losses in the upstream and downstream will vary, which will result in the variable pressure drop across the valve. So in this case which is the actual case as no one installs the valve alone, the valve authority is less than one

Let’s even explain it in deep, we have a control valve on pump discharge.

So we have a pump curve where the pump head decreases as the flow increases.

We have control valve upstream piping pressure drop and downstream piping pressure drop.

Now let’s plot the pump curve.

Then let’s plot the head losses in the upstream and downstream piping on the curve.

So at a low pump flow, the pump gives us higher head, and at the same time, we have low system losses. So we shall need the control valve to exert a high pressure drop. At low system resistance, the control valve authority will approach to one.

But when we need a high flow, the pump shall give us less head, and the system losses will increase. This will lead to a low valve pressure drop and the authority will drop.

The below is a part of "Unlock P&ID Preparation Guidelines and Common Practices". It is showing the part related to pumps maintenance and safety considerations. If you'd like to know more about P&ID preparation guidelines, you can check out the course with an exclusive discount in the bonus lecture.