Pump oversizing: Q&A with Peter Staddon

Over-sizing of Pumps is among the topics covered in the new Pumps & Valves supplement, which is included in the Jan/Feb 2013 issue of Process Engineering. For this feature we asked industry expert Peter Staddon of The Pump Company for his views on this highly topical – and complex – subject:

As a leading player in the sector, to what extent do you think ‘oversizing’ is an issue for the process industries, and in which sectors/applications is the problem biggest?

PS The oversizing of pumps is a major problem, often due to the number of people involved in the buying decision, each person tends to be afraid of under sizing the pump for the job. In most cases an additional safety margins are added to the original calculation and this can even be exacerbated by a pump application engineer taking a short cut and not checking the clients system or worse still, selling a larger pump that is in stock rather than telling the client that the better sized pump is on an extended lead time.

Why do engineers continue to specify pumps that are over-sized for the application involved, and what problems can this cause?

PS Overestimating the system head requirements is a frequent problem and often the cause of motors failure due to the actual head being much lower, the pump therefore runs to the right of the curve and if not throttled back higher flows are realised and excessive power is drawn causing overloading of motors that do not cover the end of curve
Oversizing of pumps in a ring main system can lead to an increase in the product temperature, cavitation/NPSH issues, premature bearing failures, vibration, low efficiencies and much greater power consumption

What is the role of suppliers in all this?

PS A responsible supplier has a duty to understand what the client is trying to achieve and this involves asking the correct questions about the system, the product as well as advising on the difference between the various options that the client may have as a solution. Sadly there are a number of pump suppliers that do not have suitably qualified sales and application personnel, who in the worst cases select from a brochure and do not ask the client what they are trying to achieve nor have a clear understanding themselves the ramifications of incorrect selection.
Worse still is the client who purchases direct from the internet without understanding the effects of oversizing

What are the barriers to and drivers for improvement in this area?

PS Barriers are often that a client will replace an old pump with the same size pump despite the fact that the duty may have changed, planning for expansion of the system in the future, lack of investment in variable speed drives instead of using throttling valves or bypass systems and a failure to understand that the cost of excessive power consumption far exceeds the cost of the correctly specified system from the start

Any other points?

PS Use a reputable pump supplier who is prepared to look at your system and advise on the effects of incorrect pump selection by offering options and reasons for selection, at The Pump Company “we want to have clients that come back and pumps that don’t”