The Importance of Minimum Flow Protection on Pumps
On 10 August 2016, an unhappy customer at a Refining Plant contacted us due to a leaking seal which they had just installed after the previous seal failed. We accompanied him to the plant to witness the leak. The seal was a 38mm CAPI (double seal) with plan 53B. The pumped product was Hydrocarbons, C1 to C6 at a temperature of 43°C and pressure of 1200 kPa.
Underneath the outboard seal it was wet with barrier fluid. The barrier fluid pressure was down to stuffing box pressure, and we asked them to pump up the pressure to where it should be. We looked at it for quite some time before we saw some fine mist droplets coming out. It was very fine and not easy to see and did not squirt out continually. It was however clear that there was a leak. We asked them to start the other pump.
When they switched the pumps, we noticed that they did not open the minimum flow valve of the newly started pump. We felt the pipe temperature by hand and it was cold. We immediately felt the min flow line temperature of the pump that had just been switched off. It was also cold. The production personnel confirmed that they do not open these valves when they run the pumps.
The question immediately arises – Will this have an effect on the pump?
We confirmed the minimum required flow value from the pump datasheet, went to the control room and asked the process engineer for a plot on the flow of the pump over a period of a year.
The pump datasheet shows that the minimum required continuous flow is 3,1 m³/h
The following graph is a DCS plot of the flow over a period of a year supplied by the process engineer.
DCS Plot of Flow
The operating point of the pump was then plotted against the performance curve and the minimum required suction pressure curve.
Operating Point vs. Performance Curve
Operating Point vs. Minimum Suction Pressure Curve
Conclusions
1. From the above DCS graph, plotting the flow over a period of a year, it is clear that the pump mostly operates below the required minimum flow value of 3,1 m³/h. In some cases it operates against a dead head. This operation with low flow will cause shaft deflection.
Shaft deflection is a situation which occurs when a pump operates far to the left or right of the pumps best efficiency point (BEP) on the pump performance curve. Due to high radial forces on the impeller, the shaft is deflected into one position while it is rotating. The more slender the shaft is, the more it will deflect. The calculated slenderness ratio should be smaller than a value of 2. For this shaft the slenderness ratio was calculated to be 8,9. The shaft would therefore deflect severely under high radial load conditions.
Shaft deflection causes leaks and high temperatures on sealing faces of mechanical seals. With hydrocarbons, the high temperatures will cause coking. Photos from previous seal failures, confirm these symptoms. See photos below:
2. From the above pressure-flow curve, it can be seen how the pump is operated to the left of the required minimum flow. The minimum flow line was not operated, however, if it was in operation, the pump would still operate far left of its BEP. The customer must take note of this and cannot expect a better seal life if these problems are not addressed. The ΔP is also lower that what it should be. This could be due to an impeller installed with a smaller diameter than required; however, it would not cause the seal to leak.
3. From the above minimum suction pressure curve, it can be seen the suction pressure is adequate, and not the cause of the leaking seal.
General problems experienced with minimum flow protection lines
- The lines are not commissioned as was the case with the failure described above.
- The Orifice in the line blocks up partially or fully – without anyone knowing.
- The Orifice in the line erodes away – without anyone knowing. In this situation the pump could be operating to the right of the curve, even to the extent of causing cavitation due to the higher NPSH requirement.
- Automatic recirculation valves in the lines get stuck.
- Flow control valves in the lines operate incorrectly.
It is very seldom that a minimum flow protection line is equipped with a device measuring the flow.
AESPUMP has purchased an ultrasonic flow meter which can be used to measure the flow in the line while the line is in operation. With this meter inadequate flows or excessive flows, which in both cases can lead to seal failures, can easily be detected.
It is important to point out to the customer when the problem does not lie with the seal, but with the pump or the operation of the pump. It is then up to him to decide what is the best business economics: Frequently replace the seal, or sort out the pump or operational problem.