Reliable, troublefree pump operation is the key to low-cost cryogenic liquid pumping, according to "FrostByte," a newsletter published by Cryogenic Industries, Murietta, Calif. When installing either reciprocating or centrifugal cryogenic liquid pumps, careful upfront planning can mean the difference between long-term reliability or excessive maintenance and consequent downtime, the newsletter reports, listing several considerations that apply equally to reciprocating and centrifugal pumps.

Flexible lines connecting the piping to the suction and discharge fittings must be used to take up the stress or shrinkage when the system is cold. To ensure that the flexible lines are adequate, the pump should be bolted to the pad after cool down, which is not the normal practice on reciprocating pumps. This procedure relieves any stress in the piping, allowing the pump to be practically stress-free when in operation.

The pump should be mounted in the following sequence:

1. Place pump on the pad at desired location, but do not secure in place.

2. Connect suction and discharge piping.

3. Cool down pump.

4. Bolt pump to pad.

Several suction-piping issues should be addressed when planning either a reciprocating or centrifugal pump installation. In general, good piping practices improve the net positive suction head available to the pump. When planning the installation, take into account the location of the pump with respect to the tank and the process to minimize piping runs. For the suction connection, place the pump at a location that limits the piping run to less than 5' from the tank. All pipes in the system should have a pressure rating above system-design pressure. Use as few elbows as possible to minimize liquid turbulence in the line and lessen pressure drop. The suction line should have a slight and continuously downward slope to aid in maintaining liquid flow into the suction fitting. At no point should the line rise and then drop, which creates a gas trap.

A gate or ball valve, rather than a globe valve, should be used in the suction line. An inlet strainer is needed in suction piping except for some reciprocating pumps that have a strainer built into the suction fitting. A differential pressure gauge should be used across the suction strainer.

Avoid the use of suction piping having a different diameter from the pump inlet fitting. If the diameter is too large, product flows too slowly, permitting excessive heat to leak into the fluid, which may cause the pump to cavitate. Conversely, small diameter piping increases pressure losses which reduces net positive suction head and also may cause cavitation. For reciprocating pumps, connect the suction fitting to a 6 to 8" long (maximum) flex line to compensate for expansion and contraction. Flexible lines should not be used to compensate for misalignment or poor piping installations. Also, do not use full-length flexible lines, which add considerably to pressure drop and heat leak.

If the suction piping is relatively long, insulation should be considered. Vacuum-jacketed insulation is preferred because other conventional types of insulation may accumulate moisture, resulting in loss of insulation effectiveness and possibly causing cavitation due to heat leak.

Always use sound engineering practices for a pump installation. If unsure of the proper criteria, contact ACD or ACD CRYO at (951) 696-7840, or