Minimum Safe Flow for pumps has been discussed on numerous occasions on "Cheresources". Many posts have discussed the line sizing, flow control and the scheme for common minimum flow piping from multiple pumps operating in parallel.
Let us discuss pump minimum flow as defined by "API STD 610 - Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries".
Minimum Continuous Stable Flow: It is the lowest flow at which the pump can operate without exceeding acceptable vibration limits.
Minimum Continuous Thermal Flow: It is the lowest flow at which the pump can operate without its operation being impaired by the temperature rise of the pumped liquid.
Pump operations below these points can cause shaft vibration and / or reduce the mechanical seal life both due to vibrations and high fluid temperatures.
To avoid such problems, high capacity pumps shall be provided with a minimum flow bypass with the flow controlled by a restriction orifice or a control valve. If a restriction orifice is used, the operating flow rate of the pump should be increased to account for the continuous bypass flow.
The sizing of the minimum bypass flow circuit shall be based on the higher of the two minimum flows.
If it is not possible to provide a minimum flow circulation line, then adequate instrument protection shall be provided to prevent pump operation below the minimum allowable flow rate.
Today's blog entry is more about "minimum continuous thermal flow". When there is no flow from the pump by way of a valve closed to the pumped liquid reciever the power input to the pump is converted to heat in the casing of the pump causing the temperature of the liquid to rise in the casing. To prevent this happening the "minimum continuous thermal flow" needs to be ensured through the pump when it is operating.
Most experienced process engineeers are aware that this minimum safe flow to prevent temperature rise is provided by the pump vendor. During the preliminary phase of engineering, when no vendor data is available for the minimum safe flow and the minimum safe flow recirculation line is to be sized an approximation of the minimum safe flow needs to be done. Often, this assumed or approximated value is 25 to 30% of the forward flow. Obviously after the receipt of the vendor data for the minimum safe flow, the recirculation line hydraulics needs to re-checked based on the value provided by the pump vendor.
However, it is possible to calculate the minimum safe thermal flow for general purpose centrifugal pumps based on the spreadsheet that I am attaching. However, this spreadsheet is only applicable for existing pumps where data such as brake horse power at shut-off, shut-off head and the pump efficiency at shut-off is available. In other words, the spreadsheet may be used to verify the minimum safe flow given by the pump vendor. The equations for calculating the "allowable temperature rise" and the "minimum safe thermal flow" are in USC units. However, I have programmed the spreadsheet to take inputs in SI units and give the output of minimum safe flow in both USC units (US gpm) and Metric units (m3/h).
Hope all of you find this blog entry and the accompanying spreadsheet interesting and useful. I would be happy to receive your comments and will try to answer any queries raised by you.
Download the MS Excel Spreadsheet Here:
Minimum Safe Flow for a Centrifugal Pump