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Three years ago, I posted a question about the use of spiral wound gaskets pressurized from OD to ID. The real issue I was exploring was a set of large barrel pumps that were chronically underperforming. I was speculating that the gasket stack between the outer head and inner casing was allowing leakage from discharge back to suction as a result of the configuration of the gaskets.
Refer to thread378-381741.
This question went unresolved for three years until last week. The problem was recirculation from discharge back to suction. But, it was not occurring by leakage past the spiral wound gaskets. The problem ended up being a piping error that occurred in 1994 when the pumps were originally installed. This is a large barrel pump with an axially split inner casing. The outer barrel has two case drains. The cross sectional drawing suggested that both of these drains were connected to the portion of the casing normally under discharge pressure. This drawing was wrong. One of these drain connections actually breaks through into the suction chamber inside the casing. These two case drains were piped together with a common drain line out to a single valve at the edge of the skid.
The arrangement of this case drain piping had created an accidental spill-back path for discharge back to suction. For these particular pumps, the differential pressure is in the range of 1750 psi (4,900 feet or almost 1500 meters TDH). This spill-back created two problems. First, it reduced the developed head of the pump by almost 20 percent. The other problem was a result of the fact that these pumps pass a small amount of abrasive solids in the stream. The location of the discharge drain connection was at the bottom of the case and at the largest possible diameter. This was acting like a centrifuge, concentrating the abrasive solids and returning them to the pump suction. This resulted in accelerated wear to bushings and wear rings.
A drawing error in 1994 showed an inaccurate representation of the location of the case drains. A piping designer created a drain manifold based on this inaccurate drawing. The result was 24 years of wasted energy and at least half a dozen pump overhauls that would have otherwise been unnecessary. I am estimating the cost of that error in the range of $6 million (US). The small details really do matter.
Johnny Pellin
Refer to thread378-381741.
This question went unresolved for three years until last week. The problem was recirculation from discharge back to suction. But, it was not occurring by leakage past the spiral wound gaskets. The problem ended up being a piping error that occurred in 1994 when the pumps were originally installed. This is a large barrel pump with an axially split inner casing. The outer barrel has two case drains. The cross sectional drawing suggested that both of these drains were connected to the portion of the casing normally under discharge pressure. This drawing was wrong. One of these drain connections actually breaks through into the suction chamber inside the casing. These two case drains were piped together with a common drain line out to a single valve at the edge of the skid.
The arrangement of this case drain piping had created an accidental spill-back path for discharge back to suction. For these particular pumps, the differential pressure is in the range of 1750 psi (4,900 feet or almost 1500 meters TDH). This spill-back created two problems. First, it reduced the developed head of the pump by almost 20 percent. The other problem was a result of the fact that these pumps pass a small amount of abrasive solids in the stream. The location of the discharge drain connection was at the bottom of the case and at the largest possible diameter. This was acting like a centrifuge, concentrating the abrasive solids and returning them to the pump suction. This resulted in accelerated wear to bushings and wear rings.
A drawing error in 1994 showed an inaccurate representation of the location of the case drains. A piping designer created a drain manifold based on this inaccurate drawing. The result was 24 years of wasted energy and at least half a dozen pump overhauls that would have otherwise been unnecessary. I am estimating the cost of that error in the range of $6 million (US). The small details really do matter.
Johnny Pellin
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