By Joe Cheema, Senior Project Engineer, Fluid Energy Controls Inc.
Stopping and starting is expensive. Cars last longer and get better mileage with freeway rather than city driving. Corporations leave computers running 24/7 to extend their life. Industrial gas turbine maintenance schedules are determined as much by number of starts as number of operating hours.
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But there is something that stops and starts more often than rush hour traffic in Los Angeles: the water or oil coursing through a hydraulic system. Each pump piston stroke sends a shockwave through the system at the speed of sound, hammering the fittings and lowering equipment life expectancy. Unless, that is, one uses a pulsation dampener to provide a constant pressure and smooth the fluid flow.
In most liquid handling systems, the primary source of pulsations is the pump. This applies to both hydraulic motion systems as well as chemical injection pumps. With any type of positive displacement pump—whether it uses a diaphragm, gear, piston or vane—the pump breaks down the inlet flow into a series of discrete parts. The pump then applies energy to each of these discrete parts, raising its pressure and then releasing it into the general high-pressure flow.
Since piston-type pumps are most commonly used, we will look at these in more depth, recognizing that the same principle applies to other types of pumps. Reciprocating piston pumps are an efficient means of achieving high pressures, however they also tend to produce the strongest output pulsations. While the average pressure and flow rate of the fluid remains relatively constant, it is subject to wide fluctuations, particularly in the area immediately following the pump output. The pump operates by taking a finite amount of fluid into its chamber and then rapidly compressing it. This action produces a sinusoidal pattern of fluid pressure and speed fluctuating around the average pressure and speed of the system.
As the high-speed, high-pressure fluid exits the discharge port on the pump, it creates a compression wave. That wave travels through the fluid at the speed of sound until it reaches a bend or restriction in the pipe. At that point, the joint or restriction absorbs some of the compression wave’s energy, while the rest is reflected back against the flow coming from the pump. The back-and-forth hammering this compression wave causes lowers the lifetime of the pump and the plumbing.
One way to smooth the flow is to use a double-acting pump or one with multiple pistons. Since each piston stroke reaches its maximum at a different time, this smoothes out the pressure highs and lows the same way a 12-cylinder Jaguar engine runs much smoother than a single piston lawnmower. The K Factor of a pump is a figure showing how much the flow volume varies from the average flow volume at different points of the piston stroke. A simplex, single-acting pump has a K Factor of 0.60, meaning the fluid volume at the discharge port varies 60 percent above and below the average flow. Adding more pistons lowers the K Factor to the point where, with seven pistons, the K Factor drops to only 0.02. Of course, not many septuplex pumps are in use. The more typical simplex, duplex and triplex pumps require pulsation dampeners to bring their pulsation problems under control.
Smooth Strokes
Pulsation dampeners are devices attached to the pump output that moderate the pump’s pressure and volume fluctuations. They can be attached on a nipple off the outlet line, or they can sit in line. Numerous designs are available, but the basic elements consist of a sphere containing a diaphragm or a cylinder containing a bladder. With the first design, the diaphragm is held in place by the two halves of the sphere. The diaphragm splits the interior of the sphere into two halves, one containing nitrogen and the other containing the fluid being pumped. A charging valve and a pressure meter sit on the gas side of the sphere. The other side connects to the plumbing. The cylindrical design is similar in operation, but a bladder is attached to the charging valve and the fluid flows in around the bladder.
In both cases, the gas side of the dampener is precharged to about 80 percent of the minimum allowable system pressure, so there will always be some liquid within the dampener. Because the nitrogen is more compressible than the hydraulic fluid, when the fluid is pressurized most of the fluid above the average system flow goes into the pulsation dampener, rather than creating a compression wave. Then, during the low pressure portion of the piston stroke, the gas expands to force the fluid back out of the dampener into the system, maintaining the mean flow and pressure.
Pressure Drop
Working together, the rubber’s elasticity and the gas’s compressibility eliminate more than 95 percent of the flow and pressure variations, prolonging the equipment life. This does require, however, that the system be properly designed, installed and maintained. To begin with, the correct materials must be selected for the diaphragm/bladder and the vessel. The diaphragm or bladder is typically made using Buna-N, however other materials (typically neoprene, nordel, viton, hypalon, silicon and teflon) are also available depending on the material being pumped. Type 304 or 316 stainless steel is normally used for the vessel, though other materials (including Alloy 20, Hastelloy C, polypropylene, PVDF, Teflon and Nylon) can also be used, as appropriate.
The next step is properly sizing the dampener. On a piston pump, the formula is to multiply four factors: the area of the plunger face in square inches (A), the stroke length of the piston in inches (L), the K Factor of the pump as discussed earlier (K) and the pressure factor (for example, a pressure factor of 50 would give approximately a 5 percent pulsation control). A x L x K x 50 = size of dampener in cubic inches.
For example, if you have a simplex pump (K Factor = 0.60) with a 3” diameter piston (A= 7”) and a five-inch stroke (L=6), then your minimum size for a dampener is 1,050 cubic inches. You then would select the next larger size dampener, in this case a 5-gallon (1,155 cu. in.) model.
Installation is the next step. The dampener should be placed as close to the pump outlet as possible. Depending on design constrictions, the dampener can either sit in line or off to the side, connected by a nipple. If using a nipple, select the shortest possible nipple diameter and ensure the dampener inlet is no smaller than the pump outlet pipe diameter or they will restrict the flow into and out of the dampener, reducing its effectiveness. The dampener should be precharged with nitrogen to 70 to 80 percent of the system mean operating pressure.
Finally, check the nitrogen pressure on a regular basis to ensure it is still at the pressure needed. Shut off the pump before checking the pressure, otherwise you will obtain a fluctuating reading closer to the system’s mean operating pressure, rather than the precharge pressure.
With these steps in place, you can rest assured that the fluid will flow smoothly, reducing vibration, extending pump and plumbing life and eliminating costly downtime. At least that is one less thing you will have to worry about, though you will still have to watch your blood pressure in the morning commute.
About the Author: Joe Cheema is a Senior Project Engineer for accumulator manufacturer at Fluid Energy Controls Inc. in Los Angeles.