More pointers for commercial pumps
net positive suction head
In HPAC’s December 2016 issue the article Troubleshooting Pointers for Commercial Pumps discussed some of the more common but simple reasons for pump issues that may be resolved without major work. This article serves as a continuation of that discussion with a focus on net positive suction head (NPSH) – a very important detail that must be considered (as several readers pointed out to me after the December article). Reader feedback and opinions are very much valued and appreciated–keep them coming.
As a reminder, this article will not discuss cavitation caused by air entrainment in the fluid or vortexing caused by the piping arrangement as it was discussed in December.
NPSH refers to the minimum suction head required to prevent cavitation in a pump. Look at NPSH as the head corresponding to the difference between the actual pressure at the inlet (suction side) of the pump and the fluid vapour pressure. An incorrect determination of NPSH can lead to reduced pump capacity and efficiency, severe operating problems and cavitation damage.
NPSH needs to be properly defined as two separate entities: NPSH required (NPSHR) and NPSH available (NPSHA).
The required or minimum NPSH is dependent on the design of a particular pump and is determined by the manufacturer’s testing of each pump model. The pump manufacturer can plot this required NPSH for a given pump model on performance curve and this value, expressed as feet of the liquid handled, is the pressure required to force a given flow through the suction piping into the impeller eye of the pump.
Required NPSH can also be defined as the amount of pressure in excess of the vapour pressure required by a particular pump model to prevent the formation of vapour pockets or cavitation. Required NPSH then varies from one pump manufacturer to the next and from one manufacturer’s model to another. The required NPSH for a particular pump model varies with capacity and rapidly increases in high capacities.
The available NPSH, on the other hand, is dependent on the piping system design as well as the actual location of the pump in that system. The NPSH available, as a function of system piping design and layout, must always be greater than the NPSH required by the pump in that system or noise and cavitation will result.
The available NPSH can be altered to satisfy the NPSH required by the pump if changes in the piping fluid supply level, and so on, can be made. Increasing the available NPSH provides a safety margin against the potential for cavitation. The available NPSH is calculated by using the following formula:
NPSHA = ha (+/-) hs – hvpa – hf
ha = atmospheric (open loop system) or system (closed loop system) pressure in feet. (Note: atmospheric conditions change with altitude above sea level as shown in the example in Table 1)
hs “+” = head in ft above suction centerline in open loop or positive pressure in a closed loop system in feet gauge
hs “-” = head in ft below suction centerline in open loop or negative pressure in a closed loop system in feet gauge
hvpa = vapour pressure of the fluid in feet absolute (changes based on fluid temperature – see example in Table 2)
hf = pipe friction in feet between pump suction and suction reference point. Beware, this must be calculated based on distance and piping material and size, as well as fittings and valves used in the system.
Cavitation can be defined as the formation and subsequent collapse of vapour pockets in a liquid. Cavitation in a centrifugal pump begins to occur when the suction head is insufficient to maintain pressures above the vapour pressure. As the inlet pressure approaches the flash point, vapour pockets form bubbles on the underside of the impeller vane, which collapse as they move into the high-pressure area along the outer edge of the impeller. Severe cavitation can cause pitting of the impeller surface and noise levels audible outside the pump.
A sample pump performance curve (Figure 1) includes a plot of the required NPSH for a pump model. If a pump capacity of 105 GPM is used as an example capacity requirement, reading vertically from that GPM rate shows a required NPSH of four feet for this particular model. An available system NPSH greater than four feet would therefore be necessary to ensure satisfactory pump performance and operation.
Now let’s look at an open loop system as an example to calculate the NPSHA. Use the formula and tables above to determine that. In our sample system, we are using an open cell cooling tower (fluid at 68F), an end suction pump and a centrifugal chiller at 4,000 ft elevation as shown in Figure 2, with a flow rate of 100 GPM running through 100′ of 3″ Sched 40 pipe.
NPSHA = 29.4 ft (ha) + 5 ft (hs+) – 0.78 ft (hvpa) – 2.8 ft estimated using online Friction Loss Tool (hf) = 30.82 ft
When you make a selection for the appropriate pump to be used in this application, simply remember the earlier statement that the NPSHA is greater than the pump’s NPSHR (see pump performance published data).
Like many things in life, the above represents rules of thumb and the most common applications and calculations that will aid in the majority of your troubleshooting needs. However, there are always exceptions to the rule and understanding the makeup of the principles will help you find a solution when you are faced with an exception.
Mike Miller is past chair of the Canadian Hydronics Council (CHC) and director of sales, building services Canada with Taco Inc. He can be reached by e-mail at firstname.lastname@example.org.
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May 11, 2022