In addition to selecting the proper suction pipe diameter and having adequate NPSHA, the submergence level and suction pipe configuration must be considered. Submergence level is the depth of the suction pipe inlet below the liquid surface. If an inadequate submergence level exists, an air vortex will form that extends from the liquid surface to the inlet of the suction pipe. This will introduce air into the system, resulting in either turbulent flow patterns or vapor locking of the pump. Amount of submergence required varies with velocity of the fluid. Fluid velocity is controlled by flow rate and pipe diameter. Refer to Figure 1. to determine submergence required based on fluid velocity (fluid velocity can be found in Friction Loss (Centrifugal Pumps Velocity Measured), in the column ‘‘V (ft/sec)’’).
If a system utilizes a 6-inch suction line with a flow rate of 600 gpm, suction-line velocities will be 6.6 fps and the line will therefore require approximately 3.5 feet of liquid surface above the suction-line entrance. Once the submergence level drops below 3.5 feet, an air vortex will form, causing air to enter the pump suction, resulting in a turbulent flow pattern and/or vapor lock.
A suction-line velocity of 6.6 ft/sec is ideal. Increasing the pipe diameter
to 8 inches would result in an insufficient line velocity of 3.85 ft/sec. However, most systems will require the tank to have the ability to drain lower than 3.5 feet. One solution is to install a baffle plate over the suction pipe. If a 14-inch baffle plate is installed, fluid velocities around the edge of the plate are only 1.25 ft/sec, which would allow the tank to be drained to approximately 1 foot above the suction pipe entrance. Refer to Figure 2. for an illustrated view of a baffle plate.
In addition to proper line size and submergence level, a suction pipe should slope gradually upward from the source to the pump suction. This prevents air traps within the suction line. There should be a straight run prior to the pump entrance of at least two pipe diameters in length to reduce turbulence. A smooth-flowing valve should be installed in the suction line that will allow the pump to be isolated for maintenance and inspection. If a suction hose is used in lieu of hard piping, the hose must be noncollapsing. Refer to Figures 3 and 4 for examples of accepted piping practices.
Supercharging Mud Pumps
Triplex mud pumps are often operated at speeds at which head in the suction tank is insufficient to maintain fluid against the piston face during the filling stroke. If fluid does not remain against the face, air is sucked in from behind the piston, causing a fluid void. If a void is formed, the piston strikes the fluid when the piston reverses direction during the pressure stroke. This causes a shock load that damages the triplex power end and fluid end and lowers expendable parts life. Supercharging pumps are used to accelerate fluid in the suction line of a triplex mud pump during the filling stroke, allowing fluid to maintain pace with the piston. A properly sized supercharging pump will accelerate fluid so that fluid voids and shock loads do not occur.
Triplex mud pumps normally have shock loads at speeds greater than 60 strokes per minute (spm) (when not supercharged). Without proper equipment, this would go unnoticed until the pump exceeded 80 strokes per minute, but meanwhile the shock load is damaging the pump. Supercharging requires an oversized pump with wide impellers to adequately react to rapid changes in flow required by the triplex mud pump. When sizing a centrifugal pump for a mud pump supercharging application, the pump should be sized for 1½ times the required flow rate. Therefore, if the triplex mud pump maximum flow rate is 600 gpm, the centrifugal pump should be sized for 900 gpm. High-speed piston and plunger pumps that stroke above 200 spm should be designed with a supercharging pump that produces 1¾ to 2 times the required flow rate.
Supercharging is one of the few applications in which the centrifugal pump does not have steady flow. The flow pulsates. Small impellers operating at 1750 rpm have a tendency to slip through the fluid when acceleration is needed. This is similar to car tires slipping on wet pavement. Even though it sometimes appears that the small impeller running at 1750 rpm is providing enough head, shock loading may be occurring. Supercharging pumps should have larger impellers running at either 1150 (60 cycles) or 1450 rpm (50 cycles) and should normally be sized to produce 85 feet of head at the triplex suction inlet. Supercharging pumps should be located as close to the supply tank as possible. Mounting supercharging pumps near the triplex and away from the supply tank transfers suction problems from the triplex to the centrifugal pump. If the centrifugal pump does not have a favorable supply with short suction run, it will have an insufficient supply to accelerate fluid.
Piping for supercharging pumps and triplex pump suctions should be oversized for the flow rate. Pipe should be sized so the change in line velocity during pulsations will not be over 1.5 ft/sec during the change from low flow rate to high flow rate during the triplex pulsation cycles.
Example: Triplex will be sized for 600 gpm. What pipe size should be used? The triplex does not have a constant flow rate but varies as it goes through its different crank positions: . High flow rate is 107% of average=600×1.07. High flow rate= 642 gpm. . Low flow rate is 86% of average=600×0.86. . Low flow rate=516 gpm. . Line velocity in 6-inch pipe at 642 gpm=7.29 ft/sec. . Line velocity in 6-inch pipe at 516 gpm=5.86 ft/sec. . Change in line velocity=7.29−5.86. . Change in line velocity=1.43 ft/sec.
Since the change in line velocity in 6-inch pipe is less than 1.5 ft/sec, this pipe size can be used for supercharging a triplex with an average flow of 600 gpm.
There are times when a single centrifugal pump will not meet the head requirements of an application. Two pumps can be operated in series to achieve the desired discharge head, in which the discharge of one pump feeds the suction of the second pump. The second pump boosts the head produced by the first. Therefore, if an application required 2900 gpm at 200 feet of head, one option would be to run two 10×8×14 pumps in series. Each pump could be configured with a 13-inch impeller to produce 2900 gpm at 100 feet of head. When operated in series, the pumps would produce 2900 gpm at 200 feet of head.
This type of configuration is most commonly used for extremely long discharge runs. When running pumps in series, it is important not to exceed flange safety ratings. Additionally, it is not required to place pumps within close proximity of each other. If an application had a 6-mile discharge line the first pump could be located at the supply source and the second pump could be located 3 miles away.
Parallel operation is discouraged for centrifugal pumps. If an application
exists that requires high volume and low head and volume required is greater than can be produced by a single pump, two pumps are sometimes used in a parallel configuration to meet the demand. Two pumps that produce the same TDH can be configured so that each pump has an individual suction but both pumps feed into the same discharge line. If the pumps are identical, head in the discharge line is equal to that of the pumps, but the volume is double what a single pump can produce. However, two centrifugal pumps will never have the exact same discharge head, and as wear occurs one pump will produce less head than the other and the stronger pump will overpower the weaker pump and force fluid to backflow into the weaker pump. For this reason, parallel operation is not normally recommended.
Two pumps can be configured in parallel but only one pump is operated at a time, thus providing a primary and a backup pump. The two pumps are separated by a valve in each discharge line that prevents one pump from pumping through the other. This type of configuration is perfectly acceptable and, in crucial applications, encouraged.