General suction system

This is an important part of the pump system and should be thought of as a very speeialized piping design. Considerable attention must be direeted to the pump suetion piping to ensure satisfactory pump operation. 

A pump is designed to handle liquid, not vapor. Unfortunately, for many situations, it is easy to get vapor into the pump if the design is not earefully done. Vapor forms if the pressure in the pump falls below the liquid s vapor pressure. The lowest pressure occurs right at the impeller inlet where a sharp pressure dip oeeurs. The impeller rapidly builds up the pressure, which collapses vapor bubbles, eausing cavitation and damage. This must be avoided by maintaining sufficient net positive suetion head (NPSFl) as specified by the manufacturer. 

Vapor must also be avoided in the suction piping to the pump. It is possible to have intermediate spots in the system where the pressure falls below the liquid s vapor pressure if careful design is not done. 

Therefore, the suetion system must perform two major jobs maintain sufficient NPSH and maintain the pressure above the vapor pressure at all points. 

NPSH is the pressure available at the pump suction nozzle after vapor pressure is subtracted. It is expressed in terms of liquid head. It thus reflects the amount of head loss that the pump can sustain internally before the vapor pressure is reached. The manufacturer will specify the NPSH that his pump requires for the operating range of flows when handling water. This same NPSH is normally used for other liquids. 

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It is recommended that any plant which makes or uses phosgene in large quantities should be provided with two separate scrubber systems. A fume scrubber (to handle low phosgene concentrations diluted with a large volume of inerts) and a caustic scrubber (to handle higher phosgene concentrations). The fume scrubber is a general suction service for... 

Particularly reasonably priced explosion isolation of systems involves the use of a relief pipe with which the flow direction can be diverted by 180°. It prevents flame jet ignition with precompression in constructionally protected equipment. If suction is present, explosion propagation can occur. To prevent this, the use of an additional extin-gui ing barrier or a rapid-action gate valve is necessary. If a diverter is installed where positive pressure feed is given, then the diverter is, in general, safe against an explosion propagation. 

In mechanical systems, the temperature of the available water (or coolant) to condense the refrigerant from the compressor determines the pressure level of this part of the system. Generally speaking, it is less expensive to operate at as low a pressure level on the discharge as is consistent with the suction pressure and with the physical characteristics of the refrigerant. Sometimes the cost of the refrigerant and the cost of its replacement on loss dictate that the optimum situation is not determined by the system and refrigerant s physical properties. 

The inside of the reservoir generally will have baffles to prevent excessive sloshing of the fluid and to put a partition between the fluid return line and the pump suction or inlet line. The partition forces the returning fluid to travel farther around the tank before being drawn back into the active system through the pump inlet line. This aids in settling the contamination and separating air entrained in the fluid. 

Refrigeration compressors which will probably be used on flooded evaporators (R.717 and the larger machines generally) will be rated with the suction at saturated conditions, since there will be little or no superheat in practice. Compressors for dry expansion systems may be rated at a stated amount of superheat, commonly 8 K. 

The mathematical model describing the two-phase dynamic system consists of modeling of the flow and description of its boundary conditions. The description of the flow is based on the conservation equations as well as constitutive laws. The latter define the properties of the system with a certain degree of idealization, simplification, or empiricism, such as equation of state, steam table, friction, and heat transfer correlations (see Sec. 3.4). A typical set of six conservation equations is discussed by Boure (1975), together with the number and nature of the necessary constitutive laws. With only a few general assumptions, these equations can be written, for a one-dimensional (z) flow of constant cross section, without injection or suction at the wall, as follows. 

Every year I run a chem lab and when someone is doing a vacuum filtration, suddenly I ll hear a scream and a moan of anguish, as water backs up into someone s filtration system. Usually there s not much damage, since the filtrate in the suction flask is generally thrown out. For vacuum distillations, however, this suck-back is disaster. It happens whenever there s a pressure drop on the water line big enough to cause the flow to decrease so that there is a greater vacuum in the system than in the aspirator. Water, being water, flows into the system. Disaster. 

When a booster fire water pump takes suction from a public main, the design should be such that operation of the pump at 150% of rated capacity will not reduce the public main pressure below 20 psi (137.9 kPa). Provisions against contamination of the municipal system should be made by adding a backflow device is typically required. Local agencies generally have specific requirements. Some locations do not allow direct suction from public water mains. 

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