Fluid Mechanics Solutions

PARALLEL PUMPING.

 

Whenever you intend to connect two identical centrifugal pumps into a parallel operation there are several design aspects you should consider:

* Although the combined performance of the units will usually match in head (outlet pressure), the capacity will not double unless the system head curve is so flat that the pumps head just happen to match the single pump condition. Generally, the combined capacity will be between 80%-50% of the doubled capacity. This factor should always be considered in the design.

* Each pump should be equipped off the discharge with matching increasers followed by matching check valves followed by isolation valves to the manifold to permit isolation and servicing of one of the units while maintaining operation of the other.

* If the system is intended to routinely operate as two combined pumping units, verify that a single pump operation does not allow overloading of the pump's electric motor (if so equipped). This is often the result of "runout" of the single pump, where the pump's discharge head falls low enough to permit extended flow beyond it's normal rate. This is a common occurrence with a system with a flat head curve.

* Just as with the ability to overload the motor with lower head, consideration of runout NPSH should also be examined and verified that cavitation will not occur with higher than normal flow with one pump in operation. Many common centrifugal pumps NPSHR curves will exhibit a sharp rise in required inlet head beyond 100%-125% of the BEP (Best Efficiency Point) to the end of the curve. Insure during your design that each pump can safely operate (or shutdown, if needed) should one pump drop off line, leaving the other to operate alone.

5) Finally, install accurate pressure gauges on the discharge of each pump to allow periodic testing of each separate unit in order to verify that each unit is properly contributing its fair share. Regardless of the intended match of impeller diameters, speed, and system design, it is a basic fact of pump system design that, even when new, one of the units will generally deliver slightly more flow rate than the other. This is usually more pronounced as the units wear and is often caused from disparate piping design, normal variations of construction of the pump and/or motor, or hydraulic anomalies. This is much easier to check if a flowmeter is included in the system design to provide a means of verification.

There are many different ways, cheap and expensive, to provide adequate protection for the condition you cite with the best solution and complexity usually tied to the consequence and risk of failure (what would happen if one pump did fail and how severe would the consequences be?), cost, and complexity of the control or pumping system (how do you best fit the protection device into the system?).

An inline globe-type control valve would certainly allow you to control the discharge pressure and avoid pump runout, however, to avoid the need of installing one on each pump you would need to install a larger valve on the discharge manifold and then you would be stuck with the permanent head loss from the valve at high and low flows and would need to plan that into your pump design accordingly. In most applications, the high cost of the valve itself would also become a factor.

Another solution, albeit still a somewhat expensive one, is the use of a "power-factor" sensing relay. This type of relay, available from manufacturers such as Time-Mark or Diversified, constantly senses the input power to the motor. If the input power gets too far from the normal operational setting, this indicates an abnormal operating condition, such as loss of prime, motor overload or underload, high or low voltage, phase reversal or loss, or another power supply issue and shuts the unit down, usually following a short time-delay. The advantage to this type of control is that it not only protects from the loss of one pump but many other common power related factors as well. This is the type of protection I usually reserve for high horsepower or critical service applications as the system costs well over $1,000, installed.

An inline flow switch (paddle type) is an inexpensive method, however, it is also prone to errant operation, even in normal flows, surge bouncing of the switch, leading to a short life, and difficult to hold a permanent adjustment due to hysteresis, loss of spring tension, and "switch flutter". Although you can purchase a much more reliable and accurate flow switch (fire line sensing type), you would need one with a full size paddle for reliable operation and protection, which will result in other problems, higher head loss, potential of switch flutter remains, and problems holding the adjustment over variable flows. The only time I would recommend any paddle switch is for applications such as a fire standpipe, where the flow is either "on" or "off" and there are no variable flow rates. In many of my industrial, irrigation, and even a few municipal or commercial designs, I have often used a low-pressure cutout switch that senses the lower pressure in the system from the loss of one pumping unit and then shuts down the other pump to protect both. This should be tied to a time-delay relay to prevent errant operation of the switch during normal pressure surges, line filling, normal low pressure events while starting the pumps and short power blips. Generally, a time-delay of around 15-60 seconds is adequate to protect the pump while not shutting them down by mistake, make sure the time-delay sequence is wired to shut the control circuit down to BOTH pumps and not allow any automatic restart, a restart should always be manually initiated after investigating and correcting the cause of the loss of the first pump. I hope this helps and let me know if I can be of further assistance

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