Poor Pump Design Considerations Can Limit Operations
In this case study, a reciprocating diaphragm metering pump installation was held to 25% of rated capacity.
Reciprocating diaphragm pump
By Jordan Grose
Jordan Grose is manager of Pump Services for Beta Machinery Analysis, Calgary, Alberta, Canada. Grose is a rotating machinery engineer specializing in vibration and reliability issues for reciprocating and centrifugal pumping systems including pulsation and mechanical analysis, water hammer and transient studies, small-bore piping analysis and other related design work. Beta Machinery Analysis is a market leader in providing engineering services for rotating and reciprocating machinery.
Relatively small reciprocating pumps, smaller than 47 hp (35 kW), are usually considered noncritical but they can create operational and production headaches at onshore and offshore facilities.
This article will discuss a case where a reciprocating diaphragm metering pump installation was limited to 25% of rated flow capacity.
Operators of two diaphragm amine pumps were very concerned about high vibrations around the pumps once the flow rate went above 25% of rated flow. These pumps had three fluid ends, and were flow controlled with a variable speed electric motor drive (see Photo 1).
Above 25% flow, much of the process piping — especially the auxiliary small-bore piping — would shake violently. The operator had to run both pumps to make up the process flow shortfall, thus eliminating their 100% redundant operation scheme.
This example illustrates how high operating risk is completely avoidable with adequate considerations at the design stage.
A troubleshooting team investigating this problem observed much auxiliary small-bore piping around the pumps to accomplish the triple redundant control system strategy for this facility (see Photo 2).
The auxiliary systems were most often piped high in the air and were poorly supported with flimsy pipe racks. This made for highly flexible small-bore piping arrangements with very low mechanical natural frequencies (MNFs).
The system had bladder-type pulsation dampeners installed on both the suction and discharge systems. Pressure pulsation measurements showed that the pulsation dampeners were not adequately controlling the pulsations in the suction and discharge piping at all speeds (flows) of the pumps, even though they were being properly maintained.
What was causing these problems?