Valve pulsation

Figure 14.13. Towers with reciprocating trays or with pulsing action, (a) Assembly of a 36 in. Karr reciprocating tray column (Chem. Pro. Co.), (b) Sieve trays used in reciprocating trays columns (left) large opening trays for the Karr column (middle) countermotion trays with cutouts (right) countermotion trays with downpipes for heavy phase, (c) Rotary valve pulsator, consisting of a variable speed pump and a rotary valve that alternately links the column with pairs of suction and discharge vessels, (d) Sieve tray tower with a pneumatic pulser [Proc. Int. Solv. Extr. Conf. 2, 1571 (1974)]. (e) A pulser with a cam-operated bellows.

Relief Valves/Pulsation Dampeners (Volume Bottles)/Inter-Connecting Piping/InitiaT ... 

Reciprocating pumps are those most commonly used in high performance Ic. The single-piston type usually has inlet and outlet check valves with some mechanism such as variable stroke frequency to minimize the effect of pump pulsations. Dual-piston pumps operate with the pistons 180° out of phase to minimize pulsations. For this system to work optimally, the piston units must be identical. 

Software packages are commercially available for simulation of hydrauhc transients. These may be used to analyze piping systems to reveal unsatisfactoi y behavior, and they aUow the assessment of design changes such as increases in pipe-wall thickness, changes in valve actuation, and addition of check valves, surge tanks, and pulsation dampeners. 

When a pulsation frequency coincides with a mechanical or acoustic resonance, severe vibration can result. A common cause for pulsation is the presence of flow control valves or pressure regulators. These often operate with high pressure drops (i.e., high flow velocities), which can result in the generation of severe pulsation. Flashing and cavitation can also contribute. 

Loose casing and support Pressure pulsations Vibration transmission Gear inaccuracy Valve vibration Dry whirl Blade passage... 

Never use malleable iron fittings or pipe unless the fluid is nonhazardous and the pressure not greater than 25 psig. Always use a pressure rating at least four times that of the maximum system pressure. t lso, never use cast iron or malleable iron fittings or valves in pressure pulsating systems or systems subject to physical shock. 

A general appreciation of some of the effect of gas pulsation on the performance and impact on the compressor pulsation drums, their nozzles, the piping system, and the cylinder valve performance, as well as possible other effects in some unique systems or items of equipment, can be discovered by examining the cylinder performance using an indicator card. This examination (see Chapter 12) can reveal acceptable and unacceptable performance in terms of pressure variations within the cylinder as the piston passes through its cycle. Hicks presents a helpful analysis see Figures 13-3 and 13M. 

Figure 13-6A. Influence of pulsations on valve behavior. (Used by permission Compressor and Piping System Simulation, Southern Gas Association s Gas Machinery Research Council.)...

Since discrete quantities are trapped and transferred, the delivery pressure and flow varies, as shown in Figure 32.21, which also illustrates how increasing the number of cylinders in a reciprocating pump reduces fluctuations. In the case of lobe and gear pumps the fluctuations are minimized by speed of rotation and increasing tooth number, but where, for control or process reasons, the ripple in pressure is still excessive, means of damping pulsations has to be fitted. Often a damper to cope with this and pressure pulses due to valve closure is fitted, and two types are shown in Figure 32.22. 

Non-return valves used in compressor delivery lines must be designed to withstand the pressure, temperature and pulsations of compressed air. 

Pump Pulsation or Noise Flow changes sensed by the detector as a result of pump operations, such as piston movement and check valve operation. 

Thermosyphon reboilers can suffer from flow instabilities if too high a heat flux is used. The liquid and vapour flow in the tubes is not smooth but tends to pulsate, and at high heat fluxes the pulsations can become large enough to cause vapour locking. A good practice is to install a flow restriction in the inlet line, a valve or orifice plate, so that the flow resistance can be adjusted should vapour locking occur in operation. 

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