In many instances where the sizing recommendations previously discussed are used, there is not need to install dampeners to reduce liquidpulsations. However, in most instances, it is possible to further reduce pulsations and thus reduce pump maintenance costs and piping vibration with the use of pulsation dampeners. Dampeners are recommended for all major multipump installations, unless computer analog studies indicate that they are not needed. In many instances it is cheaper to install the
dampeners than to perform the detailed engineering studies to prove that they are not needed. Dampeners can be either liquid-filled, gas-cushioned, or tuned acoustically as shown in Figure 12-1.
A liquid-filled dampener is merely a large surge vessel located close to the pump. It uses the compressibility of the liquid itself to absorb pressure pulsations and thus works better on gaseous liquids (e.g., hydrocarbons, rich glycol) than on relatively gas-free liquids. The volume of the vessel is normally recommended to be ten times the pump displacement (volume per minute). Thus, for single acting pumps:
Liquid-filled surge vessels are maintenance-free and have a negligible pressure drop associated with them. However, they tend to take up a large amount of space and are heavy due to the weight of liquid. Since the vessels must be rated for the same pressure as the pipes to which they are attached, they are expensive. Figure 12-2 shows an installation for a quintuplex pump with liquid surge suction and discharge dampeners.
Typical gas-cushioned dampeners are shown in Figure 12-3. The simplest type is merely a surge bottle with a gas-liquid interface. The high compressibility of the gas provides absorption of the pressure pulses. The gas in the vapor space is normally natural gas, which over a period of time can dissolve in the liquid. Conversely, it is also possible for gas to flash from the liquid being pumped. Thus, a level gauge is installed so that the interface position can be observed and either more gas added or excess gas vented to maintain the required gas volume. The required gas volume is given by:
The allowable pressure pulsation amplitude is somewhat arbitrary. Figure 12-4 can be used as an estimate if other information is not readily available.
Gas-cushioned dampeners are much smaller than liquid-filled dampeners but require monitoring of the interface. They have the drawback of not being practical in locations where the discharge pressure varies widely and the gas volume expands and contracts in response.
Most gas-cushioned dampeners employ a pressurized bladder to keep the gas from being absorbed in the liquid. They can have a configuration such as that shown in Figure 12-3, or the bladder can be in the shape of a cylinder such as the in-line bladder of Figure 12-5. The use of diverters in appendage-type dampeners or in-line configurations aids in attenuating high-frequency pulsations.
The size of the gas volume depends upon the bladder properties and configuration of the design. For approximating purposes, Equation 12-2 can be rewritten:
The bladder precharge pressure is normally set at 60-70% of average fluid pressure. Bladder type desurgers have the disadvantage that the elastomer can eventually wear out and need to be replaced, and is limited to applications below about 300°F. However, they are normally an economical solution and are in common use where it is not expected that frequent attention will be paid to the gas-liquid interface.
Tuned acoustical dampeners are formed when two liquid-filled vessels are connected by a short section of small diameter pipe called a choke tube. This system can be designed to have a specific resonant frequency. Pressure pulsations at frequencies above this level are attenuated considerably. They are excellent for high-frequency pulsations, can be used in high-temperature situations, and are essentially maintenance free. However, they have the disadvantages of high cost, they take up a lot of space, and have a relatively high pressure drop through the choke tube when compared to the other alternatives. For this reason they are not normally used on suction lines where NPSH may be a problem. Figure 12-6 shows a pump installation with an appendage bladder dampener on the suction and a tuned acoustical filter on the discharge. The pressure vessel contains two sections, one above the other, with a choke tube connecting them internally.
An extremely efficient type of dampener can be made by connecting two gas cushion dampeners in series with a short run of pipe that acts as a choke tube. The gas-cushion dampeners attenuate low-frequency pulsations and the choke tube arrangement serves to alleviate high-frequency pulsations. Most installations do not require this complexity.