Mark Wilson, Sr. manufacturing engineer
Schering-Plough HealthCare Products, Cleveland, Tenn.
Increase the static head, redo the suction piping
The situation sounds like an NPSH problem. The problems increase during the summer as the warmer temperatures increase the vapor pressure, and reduce the NPSHA. The problem is worst with tanks 3 and 4 due to the different piping arrangement creating unnecessary friction losses in the suction line. Using concentric reducers also contributes to the problem as entrained gas accumulates at the top of the reducer until the bubble is sucked into the pump. Another problem with the pipe setup on the inlet of pump 2: at least 10-diameters of straight pipe are required. This spool minimizes turbulence in the suction line. Some possible solutions are: lower the pumps or raise the tanks to increase the NPSHA, redo the piping between Tanks 3 and 4 to minimize friction losses, remove the concentric reducers and replace with eccentric reducers, increase the inlet pipe length feeding pump 2. One question would be, is it cheaper to fix constant seal leaks or shut down the process to correctly fix the problem? (The EPA fines for seal leaks.)
Travis DeFries, plant engineer
Archer Daniels Midland, Decatur, Ill.
Add spacer to pump 2 suction
In the summer months, vapor forms and this may be causing the cavitation. A 10-in. spacer should be installed in the suction piping to pump 2. The spacer on pump 1 is keeping the vapors from going directly into the pump suction.
Jose M. Sentmanat, consultant
Use eccentric reducers
This cavitation problem is occurring due to a vapor pocket that is created in the reducer. Summer heat is vaporizing enough toluene to accumulate in the concentric reducer and the 10-diameter spacer. This vapor is being sucked into the pump, thus lowering the NPSHA. There are two potential ways the problem could be solved. They could be used in combination or individually: 1) replace concentric reducers with eccentric reducers; 2) If concentric reducers are the only choice, then vent the vapor from the problem fittings to the storage tank vent. This will relieve all generated vapor to a safe place. If the reducers are replaced, install the eccentric reducers with the flat side on top.
Girish Malhotra, PE, president
EPCOT International, Pepper Pike, Ohio
Increase the NPSHA
The problem here is that the inlet suction pressure of the pumps is below the vapor pressure of toluene when pumping from the number 3 and 4 tanks. Assuming no differences between the piping and tanks, we can rule out these potential causes. The problem is the additional friction losses for the troublesome tanks. Each pipe bend increases the pressure loss significantly. Those losses combined with the losses because of the straight pipe lengths are sufficient to drop the available suction head at the pumps below the vapor pressure of toluene. In the summer when the product is warmer, the vapor pressure of toluene is higher and the problem is exacerbated. The problem is worse with the No. 2 pump simply because it has more bends in its piping and hence higher line losses in the inlet piping, and a lower available suction head. Several things can be done to reduce or eliminate this problem. First remove the additional bends at the Nos. 3 and 4 tanks and pipe them directly to the suction header. This in itself should eliminate the problem. Other suggested changes are to change out the piping bends to long radius bends, reduce the over all suction piping length by moving the pump closer to the suction header, lower the pump below its current elevation to increase the static head from the tanks, insulate the tanks and piping to reduce thermal gain from the sun in the summer, and increase the tank blanket pressure on the tanks. A short-term solution, which might be used to evaluate the suction head problem, would be to use the tanks only when they are at sufficient level. This level would need to be determined.
Patrick Richards, senior instrumentation designer
Irving Oil Refinery, Saint John, N.B.
Nothing is said about temperature of the toluene, other than the problems are worse in the summer; knowing the temperature of the process stream, the pressures at the tanks and at the suction of the pumps would be useful in diagnosing where the specific fixes need to occur (i.e., vapor pressures, NPSHA, areas of serious turbulence, inlet recirculation). Based on the diagram, turbulence of the process fluid is likely an issue (from tanks 3 and 4) due to the torturous path and the elbow right at the inlet to pump No. 2 does not help matters.
The possible solutions (in order of ease of implementation):
- Start running both pumps and pinch back the downstream (discharge) flow off each pump to satisfy process conditions; this backs the pumps up their curves where they should require less NPSHR and hopefully reduce cavitation.
- If the pumps can be fitted with suction inducers, cavitation can be reduced dramatically. Also, install flow straightening vanes (anti-swirl) in the elbow before pump 2 and into the line into pump 1 this should reduce the seal failure issues (to some degree).
- That the pumps seem okay on tanks 1, 2 and 5 suggest the concentric reducers into the pumps are not a big issue, but they could be contributing with inlet recirculation flow, when on tanks 3 and 4; if inlet recirculation flow is apparent, change to eccentric reducers. The eccentric reducers should be oriented with the flat side on top.
- The flow may be extremely turbulent from tanks 3 and 4 due to the elbows before reaching the main header, so piping the tank flow from 3 and 4 directly into the main header may help.
- Insulate the lines from tanks 3 and 4 and the pumps may reduce cavitation, by keeping the toluene cooler, hence changing the vapor pressure point, during the summer months.