Pump Performance Curves and Matching

a Pump to a Piping System

Fundamental Parameters

Some fundamental parameters are used to analyze the performance of a pump

Mass flow rate (Volumetric Flow rate in case of compressible fluids)

In the turbomachinery industry, volume flow rate is called capacity and is simply mass flow

rate divided by fluid density

Net head H

The dimension of net head is length, and it is often listed as an equivalent column height of

water, even for a pump that is not pumping water.

• Special case with Dout = Din and Zout = Zin we will have

By dimensional reasoning, we must multiply the net head from previous equation

by mass flow rate and gravitational acceleration to obtain dimensions of power.

Thus,

In pump terminology, the external power supplied to the pump is called the brake

horsepower, which we abbreviate as bhp. For the typical case of a rotating shaft

supplying the brake horsepower,

We define pump efficiency as the ratio of useful power to supplied power,

Performance Curve Free Delivery

The maximum volume flow rate through a pump occurs when its net head is zero, H = 0; thisflow rate is called the pump’s free delivery. The free delivery condition is achieved whenthere is no flow restriction at the pump inlet or outlet—in other words when there is no loadon the pump.

Volumetric flow rate is large

Head is zero

pump’s efficiency is zero because the pump is doing no useful work, as is clear fromprevious equation

Shutoff head

The net head that occurs when the volume flow rate is zero, V = 0, and is achieved when theoutlet port of the pump is blocked off.

H is large

Volumetric flow rate is zero

pump’s efficiency is again zero, because the pump is doing no useful work.

Best efficiency point (BEP)

• The pump’s efficiency reaches its maximum value somewhere between the shutoffcondition and the free delivery condition; this operating point of maximum efficiency isappropriately called the best efficiency point (BEP)

• The Plots of Actual Head, Total power consumption, and efficiency versus volumetric

flowrate are called Characteristic curves

The steady operating point of a piping system is established at the volume flow rate

where Hrequired = Havailable.

For a given piping system with its major and

minor losses, elevation changes, etc., the required

net head increases with volume flow rate. On the

other hand, the available net head of most pumps

decreases with flow rate, as in Figure, at least over

the majority of its recommended operating range.

Hence, the system curve and the pump

performance curve intersect as sketched in

following Figure and this establishes the operating

point.

If efficiency is of major concern, the pump should

be carefully selected

I. A new pump should be designed such that

theoperating point is as close to the BEP point as

possible

II. may be possible to change the shaft rotation speed

so that an existing pump can operate much closer

to its design point (best efficiency point).

Pump Manufacturer Performance Curves

It is common practice in the pump industry to offer several choices

of impeller diameter for a single pump casing. There are several

reasons for this to:

I. save manufacturing costs,

II. enable capacity increase by simple impeller replacement,

III. standardize installation mountings,

IV. enable reuse of equipment for a different application.

• combine performance curves of an entire family of pumps of different impeller

diameters onto a single plot Specifically, they plot a curve of H as a function of V

. for each impeller diameter in the same way as in Figure. but create contour lines

of constant efficiency, by drawing smooth curves through points that have the

same value of pump efficiency for the various choices of impeller diameter.

Conclusion

• It is clear from the performance plot that for a given pump

casing, the larger the impeller, the higher the maximum

achievable efficiency. Why then would anyone buy the

smaller impeller pump? To answer this question, we must

recognize that the customer’s application requires a certain

combination of flow rate and net head. If the requirements

match a particular impeller diameter, it may be more cost

effective to sacrifice pump efficiency in order to satisfy these

requirements.

Pump Cavitation and Net Positive Suction Head

When pumping liquids, it is possible for the local pressure inside the pump to fall

below the vapor pressure of the liquid, Pv. (Pv is also called the saturation

pressure Psat

When P , Pv, vapor-filled bubbles called cavitation bubbles appear. In other

words, the liquid boils locally, typically on the suction side of the rotating

impeller blades where the pressure is lowest.

After the cavitation bubbles are formed, they are transported through the pump to

regions where the pressure is higher, causing rapid collapse of the bubbles.

It is this collapse of the bubbles that is undesirable, since it causes noise,

vibration, reduced efficiency, and most importantly, damage to the impeller

blades.

Repeated bubble collapse near a blade surface leads to pitting or erosion of the

blade and eventually catastrophic blade failure.

It is useful to employ a flow parameter called net positive suction head (NPSH),

defined as the difference between the pump’s inlet stagnation pressure head and

the vapor pressure head,

Pump manufacturers test their pumps for cavitation in a pump test facility by varying the volume flow

rate and inlet pressure in a controlled manner. Specifically, at a given flow rate and liquid temperature,

the pressure at the pump inlet is slowly lowered until cavitation occurs somewhere inside the pump.

The value of NPSH is calculated using equation and is recorded at this operating condition. The

process is repeated at several other flow rates, and the pump manufacturer then publishes a

performance parameter called the required net positive suction head (NPSHrequired), defined as the

minimum NPSH necessary to avoid cavitation in the pump.

Pumps in Series and Parallel

When faced with the need to increase volume flow rate or pressure rise by a small amount,

you might consider adding an additional smaller pump in series or in parallel with the

original pump.

Arranging dissimilar pumps in series or in parallel may lead to problems, especially if

one pump is much larger than the other.

A better course of action is to increase the original pump’s speed and/or input power

(larger electric motor), replace the impeller with a larger one, or replace the entire pump

with a larger one.

Arranging dissimilar pumps in series may create problems because the volume flow rate

through each pump must be the same, but the overall pressure rise is equal to the pressure

rise of one pump plus that of the other. If the pumps have widely different performance

curves, the smaller pump may be forced to operate beyond its free delivery flow rate,

whereupon it acts like a head loss, reducing the total volume flow rate.

Arranging dissimilar pumps in parallel may create problems because the overall pressure

rise must be the same, but the net volume flow rate is the sum of that through each branch.

If the pumps are not sized properly, the smaller pump may not be able to handle the large

head imposed on it, and the flow in its branch could actually be reversed; this would

inadvertently reduce the overall pressure rise. In either case, the power supplied to the

smaller pump would be wasted.

Series Combination

• When operated in series, the combined net head is simply the sum of the net

heads of each pump (at a given volume flow rate).

Parallel Combination When two or more identical (or similar) pumps are operated in parallel, their

individual volume flow rates (rather than net heads) are summed

Affinity Laws• Dimensionless groups those are useful for relating any two pumps that are both

geometrically and dynamically similar. It is convenient to summarize the similarity

relationships as ratios. Some authors call these relationships similarity rules, while others

call them affinity laws. For any two homologous states A and B

• The pump affinity laws are quite useful as a design tool. In particular, suppose the

performance curves of an existing pump are known, and the pump operates with reasonable

efficiency and reliability. The pump manufacturer decides to design a new, larger pump for

other applications, e.g., to pump a much heavier fluid or to deliver a substantially greater

net head. Rather than starting from scratch, engineers often simply scale up an existing

design. The pump affinity laws enable such scaling to be accomplished with a minimal

amount of effort.

Centrifugal Pumps

Centrifugal pumps and blowers can be easily identified by their snail-shaped

casing, called the scroll

They are used in cars—the water pump in the engine, the air blower in the

heater/air conditioner unit, etc. Centrifugal pumps are ubiquitous in industry as

well; they are used in building ventilation systems, washing operations, cooling

ponds and cooling towers, and in numerous other industrial operations in which

fluids are pumped

There are three types of centrifugal pump that warrant discussion, based on

impeller blade geometry

backward-inclined blades (most common)

radial blades,

forward-inclined blades.

Backward-inclined blades

most common

yield the highest efficiency of the three because fluid flows into and out of the

blade passages with the least amount of turning.

Sometimes the blades are airfoil shaped, yielding similar performance but even

higher efficiency.

The pressure rise is intermediate between the other two types of centrifugal pumps

Radial blades (also called straight blades)

Have the simplest geometry

Produce the largest pressure rise of the three for a wide range of volume flow rates

The pressure rise decreases rapidly after the point of maximum efficiency

Forward-inclined blades

Produce a pressure rise that is nearly constant, albeit lower than that of radial or

backward-inclined blades over a wide range of volume flow rates.

Forward-inclined centrifugal pumps generally have more blades, but the blades

are smaller

Centrifugal pumps with forward-inclined blades generally have a lower maximum

efficiency than do straight-bladed pumps

• Radial and backward-inclined centrifugal pumps are preferred for applications where one needs to provide volume

flow rate and pressure rise within a narrow range of values. These types of pumps are less forgiving (less robust).

• The performance of forward-inclined pumps is more forgiving and accommodates a wider variation, at the cost of

lower efficiency and less pressure rise per unit of input power.

• If a pump is needed to produce large pressure rise over a wide range of volume flow rates, the forward-inclined

centrifugal pump is attractive.

Velocity Distribution from Centrifugal Pumps

Velocity Distribution from Centrifugal Pumps (cont.)

Velocity Distribution from Centrifugal Pumps (cont.)

Velocity Distribution from Centrifugal Pumps (cont.)