Minimum suction pressure

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). 

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

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

To prevent cavitation, it is necessary that the pressure at the pump suction be sufficiently high that the minimum pressure anywhere in the pump will be above the vapor pressure. This required minimum suction pressure (in excess of the vapor pressure) depends upon the pump design, impeller size and speed, and flow rate and is called the minimum required net positive suction head (NPSH). Values of the minimum required NPSH for the pump in Fig. 8-2 are shown as dashed lines. The NPSH is almost independent of impeller diameter at low flow rates and increases with flow rate as well as with impeller diameter at higher flow rates. A distinction is sometimes made between the minimum NPSH required to prevent cavitation (sometimes termed the NPSHR) and the actual head (e.g., pressure) available at the pump suction (NPSHA). A pump will not cavitate if NPSHA > (NPSHR + vapor pressure head). 

Vacuum Pump Type Iniet Flow Rate " m /h Single-Stage Compression Ratio Minimum Suction Pressure ... 

During the SD of a freeze-drying process less and less ice is condensed. The compressor runs a long time at the minimum pressure. Thereby the cooling of the motor and/or the lubrication can be reduced. To avoid this the minimum suction pressure of the compressor has to be limited, with the consequence that the lowest temperature to be reached is also limited. 

A variant is the micro-pipette method, which is also similar to the maximum bubble pressure technique. A drop of the liquid to be studied is drawn by suction into the tip of a micropipette. The inner diameter of the pipette must be smaller than the radius of the drop the minimum suction pressure needed to force the droplet into the capillary can be related to the surface tension of the liquid, using the Young-Laplace equation [1.1.212). This technique can also be used to obtain interfacial tensions, say of individual emulsion droplets. Experimental problems include accounting for the extent of wetting of the inner lumen of the capillary, rate problems because of the time-dependence of surfactant (if any) adsorption on the capillary and, for narrow capillaries accounting for the work needed to bend the interface. Indeed, this method has also been used to measure bending moduli (sec. 1.15). 

Refrigerant composition (refrigeration systems), 231-234, 241 speed-limited, 232-233 horsepower-limited, 233 accumulator relief valve, 233-234 minimum suction pressure, 234 Refrigerant condenser problems (evaporating problems), 240 Refrigeration systems, 227-241 efficiency, 227-229 diagnosing compressor problems, 229—231 refrigerant composition, 231—234 ... 

No. Stages Minimum Practical Absolu te Pressures, mm. Hg. Range Operating Suction Pressure, mm. Hg. Closed Test Pressure mm. Hg. 

Figure 6-32 illustrates ejector systems with large condensable loads which can be at least partially handled in the precondenser. Controls are used to maintain constant suction pressure at varying loads (air bleed), or to reduce the required cooling water at low process loads or low water temperatures [2]. The cooler W ater must not be throttled below the minimum (usually 30%-50% of maximum) for proper contact in the condenser. It may be controlled by tailwater temperature, or by the absolute pressure. 

The minimum recommended saturated suction temperature on series multistage reciprocating-centrifugal applications, column (K), represents an approximate reasonable limit on suction temperature. The temperatures shown correspond to suction pressures below atmospheric. 

Each pump should be provided with an automatic circulation relief valve set below the shut-off pressure at minimum expected suction pressure (see NFPA 20). Pumps up to 2,500 gpm (9,500 Ipm) capacity require a y4-in valve 3,000 to 4,000 gpm (11,000 to 15,000 Ipm) capacities require a 1 -in (2.5-cm) valve to circulate sufficient water and prevent overheating of the pump when operated with no discharge. Provisions should be made for discharge of the valve to a drain. Diesel-driven pumps, for which engine cooling water is taken from the pump discharge, do not require circulation relief valves. 

What-if" planning is very important. The compressor performance should be examined for a range of pressures around the intended suction pressure. Turndown is extremely important since predicting the acid gas volumes can at times be very difficult. In this respect, one needs to consider speed changes (minimum and maximum), compressor internal temperatures with volume pockets in operation, and possible gas recycle. 

Once interstage pressures have been estimated, the steam requirements for each stage can be also estimated from Figure 23-17. If the estimates indicate steam flows less than 50 Ib/hr, the design air load should be adjusted so that 50 Ib/hr of steam is required as the minimum to any stage. Normally, the atmospheric pressure discharge stage will have a suction pressure of about 150-200 torr and will require the least amount of steam. 

Maximum operating pressure 5.45 MPa Minimum operating pressure 3.72 MPa Inlet or suction temperature 185 K Input flow 105 000 m /day (1.215 m /s) Pipe external diameter 1118 mm (44 in) Insulation thickness 102 mm (4 in) Flow-section diameter 914 mm (36 in) Roughness 0.0003... 

In fig. 1, the starting-point for the previously-mentioned compressor is shovm. The translation frequency was 50 Hz, the rotational speed of the piston was 6000 rev./min., the maximum compression 13.6 bar and the minimum pressure (suction pressure) 1.26 bar. Air was used as the working medium with a dynamic vis--6... 

As a rule of thumb, the suction pressure should not fall below twice the vapour pressure equihbrium of the Hquid. At the water temperature of 15 °C, the corresponding vapour pressure equihbrium of 17 mbar corresponds to a minimum section pressure of approximately 34 mbar. 

Clean out the pump suction strainer. Check that the suction pressure gauge is operable and that the connection is not plugged. The unit engineer should have placed a red mark on this gauge showing the minimum required suction pressure to prevent cavitation on start-up. 

---