NPSHr, or Net Positive Suction Head Required, is the pressure above vapor pressure required to suppress cavitation. The NPSHr is the suction capability of the 1st stage impeller and is a function of its inlet diameter, rotating speed, inlet blade angle, and suction approach.
The NPSHr is determined through testing in a controlled environment. At constant flow points, the NPSHa is incrementally reduced until the pump total developed head (TDH) diminishes. This break-off occurs rapidly. The NPSHr is then defined by a certain reduction of the total head developed by the 1st stage impeller. Hydraulic Institute suggests a 3% head drop to indicate cavitation. The figure on below illustrates the creation of an NPSHr curve through testing.
NPSHa is the system pressure above vapor pressure defined to a specific reference point. For vertical can pump configurations, the equation for NPSHa is:
NPSHa = net positive suction head available (ft)
Pa = absolute pressure (psia)
Pv = vapor pressure @ fluid operating temperature (psia)
s.g. = specific gravity @ fluid operating temperature
Z = elevation difference from the suction source to reference point (ft)
f = friction loss from the suction source to reference point (ft)
The critical factors that determine the minimum NPSHa include:The critical factors that determine the minimum NPSHa include:
- Tank Pressure: For most vertical can pump applications the pump is taking suction from a closed tank, such as the hotwell for condensate pumps. When the tank is not pressurized, the fluid absolute pressure is equal to its vapor pressure during steady-state operation, and the NPSHa is equal to the static height minus the friction loss. If the tank is pressurized, the NPSHa will be slightly greater.
- Tank Level: The minimum NPSHa is to be evaluated based on the minimum design level in the suction tank.
- Clogged Strainer: There is usually very little friction loss in the suction runs for vertical can applications. The strainer is the biggest influence to this regard, and suction problems are normally a result of clogged strainers. Minimum NPSHa should be calculated based on an assumed maximum pressure drop across the strainer.
One Pump Runout Flow
he pump length should be established to ensure that the NPSH available at the 1st stage impeller is above the NPSH required at the one pump runout flow. “One Pump Runout” is used in systems where pumps are operating in parallel and refers to where the single pump operates when the other pump is offline. For systems having more than one (1) pump operating in parallel, it is important that the other pumps achieve maximum flow in the event of a single-pump failure to minimize lost revenues from de-rating the plant.
Pumps will always operate at the intersection of the total pump and system curves. The total pump curve for parallel operation is obtained by adding the individual pump flows at constant system backpressures (total developed head). Maximum flow will occur at one-pump runout when the single-pump head curve (H-Q) crosses the unrestrained system curve (valves wide open).
In the diagram below, 1 Pump Flow refers to where the single pump operates when all pumps are on-line. 1 Pump Run-out refers to where the single pump operates when the other pump is off-line. Since the NPSHr increases geometrically with increasing flow, it is essential that this operating mode be defined. The minimum NPSHa value must be, at a minimum, equal to the NPSHr, based on 0% head reduction, at one-pump runout.
NPSH at Suction Nozzle
To provide reliable pump operation, it is necessary to ensure that the NPSHa at the top of the suction nozzle does not fall below zero. The NPSHa at the top of the suction nozzle is a function of the suction pipe diameter and the maximum operating flow rate.
The maximum velocity head (v2 /2g) is determined from the following:
Qmax = maximum operating flow (gpm)
d = inside pipe diameter (in)
The minimum NPSHa at the top of the suction nozzle is:
Z = distance from the minimum tank fluid level to the suction centerline (ft)
ftotal = total friction loss in suction pipe including clogged strainer (ft)
r = suction nozzle pipe radius (ft)
If the NPSHa at the top of the suction nozzle falls below zero, which is especially likely to occur when there is a strainer present in the suction piping, then the fluid at the top of the nozzle can flash and cause disturbances in the flow.
NPSH at Pump Mounting
For pump designs that have a below ground suction nozzle (usually in cases where there is in
sufficient NPSHa at the top of the nozzle when located on the discharge head), it is important to make certain that the NPSHa at the pump mounting does not fall below zero.
NPSHa at the pump mounting can be calculated by using the following equation:
Z = distance from tank to mounting (ft)
f = friction loss in suction pipe (not including strainer) (ft)
Pstrainer = pressure drop across strainer (psi)
s.g. = specific gravity at fluid operating temperature
If the NPSHa at the pump mounting does fall below zero, the fluid at this location can start to boil, causing the fluid column to surge. This will have a large negative affect on the flow conditions at the impeller.
If the NPSHa at the pump mounting does fall below zero, there is an easy solution to prevent the fluid at this location from cavitating. A steel plate can be installed inside the suction can at the top of the suction nozzle. This will effectively reduce the top-of-can fluid elevation.