Piping Basics - Introduction to pumps (Part 2)

A- Basic pump principles and calculations

1-   Pump head

The pressure that a pump must put out is usually expressed in head, or the pressure generated by an equivalent height of liquid. The head required to pump a fluid between two points in a piping system can be calculated by rearranging Bernoulli's law, where: 

Hp= H2+Hf-H1

- Hp = head required for the pump [m]

- H1 = total fluid head (elevation plus pressure plus velocity) at point 1 [m]

- H2 = total fluid head at point 2 [m]

- Hf = head lost due to friction between points 1 and 2 [m]

In most pumping installations the difference in velocity head can be ignored. If Hp = 0, then the piping is in equilibrium and no pump energy is needed to obtain the desired flow. If Hp < 0 this means that the system is not in equilibrium and flow will increase, causing Hf to increase until Hp = 0 and equilibrium is established. If increased flow is not desirable, a restriction must be placed in the pipe to cause an artificially high friction drop.

2-   Pump Power

The hydraulic power which is also known as absorbed power, represents the energy imparted on the fluid being pumped to increase its velocity and pressure.

The input power delivered by the motor to the pump is called brake power. The difference between the brake horsepower and hydraulic power is the pump efficiency. The brake power may be calculated using the formula below:

3-   NSPH

Each pump requires a certain minimum pressure at its suction flange to assure that no vapor is flashed between the pump suction and the cylinder or entrance to the impeller vane. If a lower pressure is supplied, vapor could be liberated by the liquid in the form of small bubbles that would then collapse in an "implosion" as the liquid is pressured in the pump. This is called cavitation and results in noise, vibration, greatly increased wear, and reduced pressure or throughput capacity.

The definition of NPSH is the net pressure above the vapor pressure of the liquid being pumped. When a liquid's pressure falls below its vapor pressure, gas is flashed.

Since cavitation is first and foremost the formation of gas, we can avoid cavitation by assuring that the liquid's pressure does not drop below its vapor pressure anywhere as it passes through the pump. The minimum pressure required at the pump flange is called the Net Positive Suction Head (NPSH). It is specified by the manufacturer for each pump.

In conclusion, this is what you need to know about both terms of NPSH:

The Available NPSH (NPSHA): a measure of how close the fluid at a given point is to flashing, and so to cavitation. Technically it is the absolute pressure head minus the vapour pressure of the liquid.

The Required NPSH (NPSHR): the head value at the suction side (the inlet of a pump) required to keep the fluid from cavitating, it is provided by the manufacturer).

Thus, NPSHA must always be bigger than NPSHR to avoid cavitation.

The required NPSH is higher for reciprocating pumps due to valve and fluid acceleration losses within the pump and in many high-head applications requiring a reciprocating pump the NPSH available may not be sufficient for a straightforward pump choice, thus in these cases low NPSH centrifugal pumps are sometimes used as "charge" pumps to feed the suction of the reciprocating pump.

B-  Multiple centrifugal pump installations

In designing multiple centrifugal pump installations, it is necessary to keep in mind the interaction between the pump curves and the system curve. That is, throughput cannot be doubled by adding an identical pump in parallel, and head is not doubled by adding an identical pump in series. The effect of adding two identical pumps in parallel can be seen in the figure below. Curve A is the pump curve for one pump. Curve B is constructed by doubling the flow rate at a given head to show how the pumps behave in parallel operation. Curve C shows a system curve where the addition of the second pump adds only about 50% to system throughput. Curve D shows a steeper system curve where the system throughput is only increased about 20%.

The figure below shows the effect of installing two pumps in series. Curve A is the head-flow-rate curve for one pump. The combined curve for both pumps, B, is constructed by doubling the head of Curve A at each value of flow rate. The benefit of the additional pump can be seen by inspecting the intersection of the system curves, C and D, with the pump curves.

The choice of whether to add an additional pump in series or in parallel is illustrated by Figure 11-3. If the system curve is shallow, more throughput is obtained from parallel operation. If the system curve is steep, more throughput can be obtained by series installation.

C-  Pump priming

Pump priming is the process of removing air from the pump and suction line.

In this process the pump is been filled with the liquid being pumped and this liquid forces all the air, gas, or vapor contained in the passage ways of pump to escape out. Priming maybe done manually or automatically. Not all pumps require priming but mostly do. There are Self Priming Pumps and also some layout situations where priming is not required.

Priming a pump is probably the first and one of the most important thing one should do before operating it. Not priming a pump or not doing it properly makes majority of pump problems. Any problem in pump due to lack of priming may cause financial impact due to pump maintenance and the downtime of piping system due to a malfunctioning pump.

Priming reduces the risk of pump damage during start-up as it prevents the pump impeller to becomes gas-bound and thus incapable of pumping the desired liquid.

For reliable operation, pumps must first be primed; that is, air or gases to be expelled from the suction and impeller eye area and replaced with liquid to be pumped. The pump would not function properly when not completely filled with liquid. Along with compromised performance, not priming the pump and allowed to run without fluid, it will overheat the pump system and there will be a danger of damage to critical internal pump components.

In principle, all Positive displacement pumps are self-priming. In particular, this includes different type of rotary and reciprocating pumps. The priming of Positive displacement pump is required only at the time of first starting as under dry running conditions the pump may overheat. But in a Centrifugal pump (except self-priming pump) priming is required in starting after every shutdown.

Priming isn’t required in the following cases:

-       Pump is submerged (Submersible or Vertical Sump Pumps).

-       Pump is at a lower elevation than the supply and this ensures that pump suction will be completely filled with liquid at all times (known as “Flooded Suction Condition”).