Q. What information is available regarding NPIPA and NPIPR for rotary pumps?
A. Net positive inlet pressure available (NPIPA) is the algebraic sum of the inlet and barometric pressure minus the vapor pressure of the liquid at the inlet temperature:
This value must be equal to or greater than the net positive inlet pressure required (NPIPR) as established by the pump manufacturer for the speed, pressure and fluid characteristics. Otherwise, the rate of flow will be reduced, and operation may be noisy and rough because of incomplete filling of the pump. This condition may damage the pump and the associated equipment.
Many rotary pumps can stably and quietly operate at rate of flow reductions of 20 percent to 35 percent because of low NPIPA with no ill effect. Many services use this reduced rate to operate high-vacuum systems for the extraction of gas or light liquids.
NPIPR is the pressure required above liquid vapor pressure to fill each pumping chamber or cavity while open to the inlet chamber. It is expressed in bar (psi). NPIPR is sometimes called NPSH3 for rotodynamic pumps.
Many liquids handled by rotary pumps have an unpredictable or very low vapor pressure. Most of these liquids have entrained and dissolved gas (frequently air) as well. The practical effect of dissolved and entrained gas is to increase the NPIPR to suppress the symptoms of cavitation. While true cavitation occurs if the liquid reaches its vapor pressure during the filling of the pumping cavities, most of the cavitation symptoms will be exhibited before reaching liquid vapor pressure. This is largely because the entrained and dissolved gas expands when subjected to reduced pressure. Because the level of dissolved gas is a function of the liquid and its temperature and the level of entrained gas is a function of system design and operation, NPIPR for a rotary pump is difficult to establish with precision.
NPIPR tests are normally conducted by the manufacturer in a test environment that minimizes entrained gas using a test liquid of negligible vapor pressure. NPIPR is established at the first indication of the following:
- A 5-percent reduction in output rate of flow at constant differential pressure and speed
- A 5-percent reduction in power consumption at constant differential pressure and speed
- The inability to maintain a stable differential pressure and speed
- The onset of loud or erratic noise when this criterion is previously agreed upon by all parties
For more information about rotary pump behavior, see ANSI/HI 3.1-3.5 Rotary Pumps for Nomenclature, Definitions, Application and Operation.
Q. How are rotodynamic vertical pumps with hollow-shaft drivers coupled to the motor?
A. In the hollow-shaft configuration, there is no external shaft extension. The top pump shaft, called the head shaft, extends through the driver shaft (also known as the quill), which is hollow, and is coupled at the top with a key and adjusting nut arrangement (see Figure 22.214.171.124.4.2). The coupling is located within the motor, under the motor’s drip cover. This coupling permits the shaft to be adjusted to compensate for the tolerance stack-up of the pump rotor and casing components and to provide the desired axial running clearance for the impellers. This clearance is normally specified by the manufacturer and is determined by mechanical and efficiency considerations, as well as thermal and pressure elongation expectations of the column and shafting.
This hollow-shaft arrangement provides optimum access to the head shaft adjusting nut where impeller lift is sensitive, such as on deepwell pumps. A single-piece head shaft is typically used outside deepwell pump installations in which headroom clearance for disassembly is not limited. Hollow-shaft motors are coupled to the pump head shaft with either a self-release or bolted coupling.
A self-release coupling is a device that can be used to keep a pump’s line shaft from unscrewing because of torque reversal and protect the driver from damage. Should a torque reversal occur, which might take place if motor leads were wired incorrectly, the driver coupling will lift and disengage the pump shaft. Self-release couplings should not be used on motors subject to upthrust. Bolted couplings rigidly connect the pump line shaft to the motor.