Problem-Free Pumping of Cryogenic Liquids
When installing cryogenic liquid pumps, careful upfront planning can help ensure long-term reliability and prevent excessive downtime.
Reliable, trouble-free pump operation is the key to low-cost cryogenic liquid pumping when installing either reciprocating or centrifugal cryogenic liquid pumps. Issues such as mounting, suction and discharge piping, gas-phase return piping, relief valves, and instrumentation and controls should be addressed before and during installation to avoid problems later on.
Centrifugal pumps require flex lines on both the suction and discharge nozzles to absorb the stress or shrinkage when the system is cold (figure 1). These flex lines - and the associated field piping - are large and strong. When these lines are cooled to cryogenic temperatures and shrink, they can exert heavy forces on the centrifugal pump nozzles and possibly distort the pump housing. Therefore, it is advisable to cool the pump to cryogenic temperature prior to bolting it to the pad, as this relieves any stress in the piping and allows the pump to be practically stress-free when in operation.
The pump should be mounted in the following sequence:
Place the pump on the pad at the desired location, but do not secure it in place.
Connect the suction and discharge piping.
Cool the pump.
Bolt the pump to the pad.
Reciprocating pumps use flex lines on both the suction and vapor return nozzles (figure 2). These flex lines and the associated field pipes are rather small, so the shrinkage does not exert excessive force on the pump nozzles. Also, the reciprocating pump skids are so heavy that the shrink force exerted by the flex lines probably would not move the pump skid even if it were not bolted to the pad. Flex lines are never used on reciprocating pump discharge lines. Pump discharge piping from the pump skid to the vaporizer (or the first pipe anchor support) should be installed with long radius bends to compensate for shrinkage.
centrifugal pump installation. In general, good piping practices improve the net positive suction head (NPSH) available to the pump. Careful piping design therefore can enhance system performance.
When planning the installation, take into account the location of the pump with respect to the tank and the process so as to minimize piping runs. For the suction connection, the pump should be placed at a location that limits the piping run to less than 5' (1.5 m) from the tank. All pipes in the system should have a pressure rating above the 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. Except for reciprocating pumps that have a strainer built into the suction fitting, an inlet strainer is needed in suction piping.
Avoid using suction piping that has a different diameter from the pump inlet fitting. If the diameter is too large, product will flow too slowly. A slow flow will permit an excessive heat leak into the fluid, which might cause the pump to cavitate. Conversely, small-diameter piping increases pressure losses, which reduces NPSH and also can cause cavitation.
For reciprocating pumps, connect the suction fitting to a 6 to 8" (15 to 20 cm) long maximum flex line to compensate for expansion and contraction. Flex lines should not be used to compensate for misalignment or poor piping installations. Also, do not use full-length flex lines, as they 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 might accumulate moisture, resulting in a loss of insulation effectiveness and possibly causing cavitation due to heat leakage. For operating cycles that are infrequent and of short duration, conventional insulation might be detrimental because the mass of insulation must be cooled down each time the pump is operated.
Whenever possible, pump suction piping should be separate from other liquid lines. If other pipes must be connected to the pump’s suction pipe, a valve must be placed directly adjacent to the connection to prevent a dead-leg - a void where liquid can vaporize. These vapors add heat to the fluid and could cause bubbles to flow into the pump, which might cause cavitation.
Gas-Phase Return Piping
To eliminate trapping gas, the gas-phase return line for a reciprocating pump should be sloped gently upward toward the tank. Its diameter should match the fitting on the suction adapter, and either a gate valve or ball valve is required.
For centrifugal pumps, a gas-phase recirculation line is needed on the discharge side for pump cool down and startup. A globe valve should be installed in this line to allow throttling during startup. A discharge control valve, downstream of the recirculation valve, must be located as close as possible to the pump to control the flow so that it is on the “pump’s curve.” Allowing the pump to start against no restriction, or to fill a large volume before control is established, will cause severe cavitation and likely the inability to “catch prime.”
A relief valve must be used to prevent overpressure from vaporized trapped-liquid when the suction and gas-phase return valves are closed simultaneously. When a relief valve is installed in the gas-phase return line, it is not necessary to install a second relief valve in the suction line.
For both reciprocating and centrifugal pumps, the discharge piping should match the discharge fitting size and should be rated higher than the pressure required by the application. A check valve should be installed in the discharge line to prevent backflow. A pressure-relief valve must be installed in the discharge piping, and centrifugal pumps must have a discharge control valve, as noted previously.
Pressure-relief valves must be used to prevent overpressurization in all pump installations, but particularly for any installations in which cryogenic fluids are used. Ambient heat leak will vaporize trapped liquid and cause a large pressure increase if the fluid cannot expand. Relief valves must be used at all potential liquid trapping points, particularly between a reciprocating pump’s discharge and a downstream valve, between isolation valves, and between an isolation valve and an upstream check valve.
Pumps must be instrumented for proper control and maintenance. At the very least, a discharge pressure gauge must be used to control the operation within the pump’s design limits.
For unattended operation, fully automated systems are available to control the operation and shut down the pump if a problem such as cavitation, seal leak or overpressurization occurs.
The issues discussed in this article provide general guidance for pump installations. However, each installation is unique. Always use sound engineering practices to ensure a successful pump installation.