| Sizing Pulsation Dampeners for Reciprocating Pumps |
|
| Written by Jack Boteler and Larry Wende, Flowguard USA, and Peter Jennings, Flowguard Ltd. | |
|
Page 2 of 2
Sizing Pulsation DampenersLet's begin by defining the pump details required to properly size a pulsation dampener. We will use these values in a sample calculation to help clarify the process.
Gas Volume Required = __________Pump Displacement_________ % Pre-Charge x LOD x Pump Constant By multiplying the cross sectional area of the piston by the stroke length, we determine the pump displacement per stroke to be 187-mL or 0.187-l. We recommend that the gas pre-charge pressure be set to 80 percent of system pressure. Lower pre-charge pressures may be specified elsewhere, but our experiences show that this is a low enough pressure to allow the membrane to move freely during operation while maximizing the gas volume. We will use 0.80 in the formula as the "% Pre-Charge" for 80 percent. The level of dampening (LOD) in this example is 0.05 (5 percent peak-to-peak residual pulsations). The result of the previous calculation is then divided by a constant. As noted previously, the constant is a function of pump configuration. We use a conservative 1.5 for simplex pumps, 2 for duplex pumps, and 7 for triplex pumps. Remember-if the fluid is compressible, then the constant may have to be adjusted downward. Fluid volumes above mean are well within the range of these constants. The fluid pulse above mean flow from a simplex pump, for example, is about 60 percent. When we divide full stroke displacement by 1.5 the result is a conservative 67 percent. The divisor 7 that we use for triplex pumps allows for a nominal 14 percent fluid volume above mean. While 14 percent is far above the actual 4 percent produced by triplex pumps, the higher volume is an allowance for practical reasons, specifically size and nozzle limits. Otherwise, the result would be a very small dampener relative to pump size. Gas Volume Required = ______0.187_____ = 0.668 Liters of Gas 0.80 x .05 x 7 Influences of Changing Temperatures and PressuresRanges of (process) temperature and pressure must be considered in any sizing calculations for pulsation dampeners. Compensations must be made for temperature variations, which affect gas density, and dynamic variations in system pressure, since sizing is based on a set pre-charge pressure. The objective is to select a dampener that is adequately sized to handle a range of operating pressures with a single pre-charge pressure. Remember that the gas pre-charge pressure should always be based on the minimum operating pressure as the pulsation dampener will have no effect when the system pressure is below the pre-charge pressure. In instances of either (or both) temperature and pressure variation, we compensate by multiplying the result of our original calculation by the ratio of minimum and maximum temperature and pressure extremes. Initial calculation: 0.668 liters Compensation: 0.668 x (Tmax / Tmin) x (Pmax / Pmin) Changes in ambient temperature can also influence gas density, but they're generally disregarded for the purposes of pulsation dampener sizing. It is usually sufficient to make seasonal adjustments to pre-charge pressures, if necessary. Temperature and pressure calculations are recommended to be done using absolute values (Kelvin for temperature and BarA or PSIA for pressure). Influences of Fluid CompressibilitySome fluids are highly compressible, such as cryogenics, olefins, liquefied gases, anhydrous ammonia, etc. In these instances, the benefit of lower pulsations from multiple piston pumps may be somewhat compromised. Fluid compression occurs during the leading edge of the (eccentric) crank angle. Given sufficient pressure and a high enough compressibility factor, there may be little or no overlap of pulses at all-in which case, adjustments have to be made and pulsation dampeners with larger gas volumes should be selected. Advantages of Pulsation DampenersBy installing a properly-sized pulsation dampener, users can reduce or eliminate pipe shake, vibration and noise. The result is a continuous flow of product which is required in many metering, mixing and spraying applications. Reduced pressure pulsations minimize long-term damage to instrumentation and pump components while improving the accuracy of many flowmeters and increasing pump efficiency.
Jack Boteler is the President and Larry Wende is the Product Manager for Flowguard USA, 9009 N. Loop East Suite 165, Houston, Texas 77029, Phone: 800-673-5186, Fax: 713-400-3368, www.flowguardusa.com Comments (0)
 Write comment
 |
Subscribe to this comment's feed
