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Production Plant Problem # 2 - Inconsistent Pump Flow

From my previous post, I stated about having three problems in the plant while starting and stopping it. I'm going to share our experience on problem # 2 first. I'll post problem # 1 in a few days time. There's no particular reason for me to post about problem # 2 first instead of problem # 1. I just think problem # 2 is more interesting and I learned a lot from it.

Problem # 2 – Inconsistent pump flow

The pumps below a buffer tank could not deliver the required flow rate that they are supposed to. The flow rate was extremely slow and the discharge pressure was not consistent at all. The pressure went crazy up and down from 0.5 to 5 bars. We tried to adjust and play around with the pump in order to get the desired flow rate.

Slow flow rate means lower throughput, hence lower production. In actual, if we keep on running with the low flow rate, production will be 50% less. We could not afford that to prolong. We have heavy shipment ahead and we need to immediately rectify the pump problem.

There were all together 3 pumps in a row of similar motor kW and pumping capability. Initially we thought all three pumps were having some problem (or the same problem). We asked the maintenance fitters to check, service and replace the mechanical seals. We checked the pump pressure and found it was OK because the discharge line pressure can go up to 4 – 6 bar. There’s no way the pump is having a problem.

Then we thought it was Net Positive Suction Head (NPSH) problem. We increase the level of oil inside the buffer tank so that the NPSH will increase. That didn’t work either. The flow rate was still low and the discharge pressures were inconsistent.

By this time, we suspected the pumps were experiencing serious air-lock. Air-lock is a situation where the pump could not pump efficiently and effectively due to some disturbance from air turbulence inside the pump suction line. The pump discharge line pressure inconsistency might be due to the air-lock problem. The question now is where the air is coming from? We decided to thoroughly check the pump suction line to search for any hole/leak (which air may easily enter because the tank is operating under vacuum). We removed the insulations covering the suction line, checking and inspected it, but still we could not see or detect any leak (because the tank and pipe was under vacuum, which means oil would not pour/come out from the leak due to vacuum holding it). Time was running fast but we have not yet settled the problem. We began feeling the pressure coming from our superior/management.

After almost one day running low, we made a more drastic and radical move. We hold the plant and braked vacuum – stopped production. We waited for a while. At first, we noticed nothing. The pump suction pipeline looked fine. After 15 minutes, we were caught by surprised on what we saw! Droplets of oil came out from the tank insulation. At first, it was little, but then there were more and more oil pouring out. This confirmed that there is a leak somewhere underneath the tank insulation. We cautiously removed a small portion of insulation covering the tank where the oil came out (because the oil and the surface was extremely hot (260oC)).

After successfully removing the insulation, we saw oil pouring out from the welding joint between the tank and the pipeline. We looked at each other, wandering how we are going to resolve the already identified problem. We cannot simply weld the leaking point because there traces of vacuum was still inside the tank and system. If the maintenance fitter welds the leak point, we fear spark may enter the tank (and system) and trigger fire inside the tank, pipeline and other vessels. At the same time, it is impossible to weld the leak point because oil was coming out from the tank. Should we drain the balance oil in the buffer tank? If yes, that would cost a lot of time, estimated half a day. Just imagine the downtime we already faced and add up another half a day for repair work and starting up the plant again. We have to think of a better and faster way to weld the leaking point.

Finally we came out with a very risky idea but can solve the problem faster. We have to maintain a slight vacuum inside the buffer tank just to hold the oil from pouring out from the leaking hold.

To be continued in a few days time...

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posted by zaki yamani @ 11:33 PM,


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The Author


I’m Zaki. I used to be a project, process and chemical engineer. Few years ago I successfully became a Chartered Engineer (IChemE) and Professional Engineer (BEM). I'm now employed as a chemical engineering educator/researcher/consultant. Hope you like reading my blog. I welcome any feedback from you. My email: zaki.yz[alias]gmail.com. TQ!

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