Suction pressure

Pump Suction. The net positive suction head required (NPSHR) affects the resistance on the suction side of the pump. If it drops to or near the vapor pressure of the fluid being handled, cavitation and loss of performance occurs (13). The NPSHR is affected by temperature and barometric pressure and is of most concern on evaporator CIP units where high cleaning temperatures might be used. A centrifugal booster pump may be installed on a homogenizer or on the intake of a timing pump to prevent low suction pressures. 

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

Suction pre.s.sure and temperature. Overall process requirements should be considered. Selection is usually governed by the minimum suction pressure required (the highest vacuum). 

The performance of a centrifugal compressor (discharge to suction-pressure ratio vs. the flow rate) for different speeds is shown in Fig. 11-87. Lines of constant efficiencies show the maximum efficiency. Unstable operation sequence, called surging, occurs when compressors fails to operate in the range left of the surge envelope. It is characterized by noise and wide fluctuations of load on the compressor and the motor. The period of the cycle is usually 2 to 5 s, depending upon the size of the installation. 

The suction pressure generated is equal to the liquid head difference across the annular baffle as magnified by G. The maximum suction that can be realized, assuming neghgible vapor pressure, is I atm. For example, with a differential liquid level of 2 cm (0.79 in) of water across the annulus under 500g, the suction generated by the siphon is 0.98 atm. 

Suction pressure is the pressure at the pump s suction nozzle as measured on a gauge. The suction pressure is probably the most important pressure inside the pump. All the pump s production is based on the suction pressure. The pump takes suction pressure and converts it into discharge pressure. If the suction pressure is inadequate, it leads to cavitation. Because of this, all pumps need a gauge at the suction nozzle to measure the pressure entering the pump. 

This is the pressure at the pump discharge nozzle as measured by a gauge. It is equal to the suction pressure plus the total pressure developed by the pump. 

An easy way to understand NPSHr is to call it the minimum suction pressure necessary to keep the pumped fluid in a liquid state. 

Pgs = the suction pressure gauge reading taken at the pump centerline and converted into feet of head. 

Hi = Inlet head, or the losses expressed in feet that occur in the suction throat of the pump up to and including the eye of the impeller. These losses would not be registered on a suction pressure gauge. They could be insignificant, or as high as 2 feet. Some pump manufacturers factor them into their new pumps, and others don t. Also, changes occur in maintenance that may alter the Hi. If you don t know the Hi, call it a safety factor of 2 feet. 

In this automatic thrust balancing system, the pressure behind the compressor wheel is controlled to a value between the compressor suction pressure and the wheel peripheral pressure. As the expander inlet pressure increases above the compressor suction pressure level, the resulting thrust force pushes the compressor wheel, and hence the rotor system, towards the compressor suction. In the reverse situation, when the pressure behind the compressor wheel is reduced below the wheel peripheral pressure level, the rotor system moves toward the expander. 

The required suction pressure to the expander is not available until the oxidizer tower reaches design pressure. With two compressors in operation this takes approximately 15 min. Since the expander wheel is mounted on the compressor pinion, enough flow must be available at startup to prevent overheating. The calculated windage loss of the expander wheel (40 hp) requires about 16,000 Ib/lir of flow to prevent overheating. 

Figure 16-7 shows the vibration spectrum with the speed and suction pressure kept constant but with a small 20psig increase in discharge pressure. Notice the large increase in the 9000 rpm component from 0.2 to 1.5 mil (0.0127-0.0381 mm). A further small increase in discharge pressure would have increased the subsynchronous vibrations to more than 1.0 mil (0.0254 mm) and wrecked the unit. 

When the suction pressure was raised by some 50psig (3.45 Bar) while maintaining the same discharge pressure, the unit regained its stability with the elimination of the subsynchronous component as shown in Figure 16-8. The subsynchronous instability in this machine was the result of aerodynamic excitation of the rotor systems occurring at a critical pressure rise across the machine of 770 psi differential (500-1270 psig). 

The correct flow to use is the compressor suction. However, a flow element such as an orifice in the compressor suction can rob inordinate horsepower. Therefore, sometimes the discharge flow is measured and the suction flow computed within the controller by using pressure measurements. Other times the compressor intake nozzle is calibrated and used as a flow element. The correct AP to use is the discharge minus the suction pressure. 

Figure 1 provides a convenient comparison of capacity and suction pressure ranges for typical commercial... 

Be sure to use the atmospheric suction pressure at the site. Normal blower ratings are given at sea level. 

Suction temperature Discharge temperature Suction pressure Discharge pressure Isentropic exponent Specific gravity Percent clearance ... 

The terms pressure ratio and volume ratio are used interchangeably in the literature on these machines. To prevent confusion, volume ratio t , is defined as the volume of the trapped gas at the start of the compression cycle divided by the volume of the gas just prior to the opening of the discharge port. Pressure ratio is defined, in Equation 2.64, as the discharge pressure divided by the suction pressure. Their relationship is given in the following equation. 

It is desirable to have additional axial-load control on the multistage compressor. A balance piston, also refeired to as the balance drum, can be located at the discharge end (see Figure 5-46). The balance piston consists of a rotating element that has a specified diameter and an extended rim for sealing. The area adjacent to the balance piston (opposite the last stage location) is vented, normally to suction pressure. The differential pressure across the balance piston acts on the balance piston area to develop a thrust force opposite that generated by the impellers. The pressure on the... 

In order to maintain gas purity, it is desirable to select a positive suction pressure in order to minimize air leakage into the loop. Apart from the need to minimize inlet density and, hence, test power, the selection of test inlet pressure is a matter of convenience. 

ESTIMATING PUMP HEAD WITH NEGATIVE SUCTION PRESSURE... 

Figure 5. Diagram for determining head for a pump with negative suction pressure.

When gravity is insufficient to induce flow, the pressure of the atmosphere is allowed to act on one side of the filtering medium, while a negative or suction pressure is applied on the discharge side. This type of filtering device is referred to as a vacuum filter. The application of vacuum filters is typically limited to 15 psi pressure, although there are applications where this value can be exceeded. Note... 

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