Pumps are one of the most commonly used and important equipment in process industry as they provide necessary driving force to transport fluid from one place to another and/or to increase the pressure as required by the Process. However, they per se cannot deliver a desired capacity and in general in a process plant, the capacity must be fixed as per downstream system’s requirement.
Therefore, it is necessary to provide a pump with capacity control means to ensure that it delivers the capacity to meet varying requirement of downstream system. Capacity control methods can be categorized separately for centrifugal and positive displacement pumps. For centrifugal pumps, various capacity control methods are-discharge throttling, variable pump speed and capacity by-pass. Whereas, for positive displacement pumps, they are-capacity by-pass, variable stroke length and variable speed.
Although discharge throttle is the most commonly used method for capacity control of centrifugal pumps, it can result into significant loss of energy if the pump capacity is large enough and plant or the system has to run at below 100% capacity frequently. In such situations, capacity control through variable speed could be a preferred choice. However, it could add to fixed cost. The final selection of most suitable means of capacity control is a trade-off between the fixed cost in form of variable frequency drive (VFD) and the energy saving benefits.
For positive displacement pumps, by-pass is the common means of capacity control due to its simplicity and low cost. However, similar considerations as above could warrant use of variable speed as capacity control method. For reciprocating pumps, capacity control through variable stroke length could be an effective means where there are frequent and wider capacity variations.
Date of Publication: July 10, 2017
About Author:
Satyendra Kumar Singh, B.Tech. (Chemical Technology) + M.B.A., is proprietor of Satsha Management Services-an award winning design engineering and management consulting company (www.satshamanagement.com). He possesses approximately 30 years’ experience in engineering consultancy in process and energy industries. Satyendra has authored several papers on energy, business and management, which have been published in some renowned journals/magazines such as ‘Chemical Engineering’, ‘Process Worldwide’, ‘Modern Manufacturing India’. He may be reached at satyendra.singh@satshamanagement.com, Ph. +919811293605.