Troubleshooting seals, with no apparent cause for the problem

TROUBLESHOOTING SEALS, WITH NO APPARENT CAUSE FOR THE PROBLEM ST019

These problems are the ones that drive you crazy. No matter how hard you look; the solution keeps evading you. Over the years I have collected quite a few examples. I offer some of them for your enjoyment and maybe, in the process, they will help you solve the “un-solvable”

Cavitation problems

  • The pump cavitated every time it rained.
    • Solution: The product temperature would cause it to vaporize very close to ambient pressure and when it rained atmospheric pressure dropped enough to cause the problem.
  • The pump never cavitated in the summer months, only during the winter when everything was cooler.
    • Solution: The tank vent froze during the winter months causing the pump to pull a partial vacuum in the tank.
  • The cavitation started suddenly with no warning.
    • Solution: A plastic pipe liner collapsed at the suction side of the pump. A pinhole in the liner allowed the corrosive product to corrode the pipe behind the liner.
  • The cavitation started after the packing was converted to a mechanical seal. A careful inspection showed that the seal was not leaking air into the suction.
    • Solution: The pump had speeded up (increased the rpm) when the packing was removed because of less drag or friction on the pump shaft. This increase in speed and capacity required a higher net positive suction head required (NPSHR) causing the cavitation.
  • The cavitation kept getting worse with time, nothing obvious had changed in the system.
    • Solution: The product had formed a coating on the inside of the suction pipe increasing the pressure drop and resulting in a loss of suction head.
  • The cavitation only occurred when there was a higher head at the suction of the pump and stopped cavitating when the level fell in the tank, just the opposite of what should have happened.
    • Solution: The pump was pumping to a fixed discharge head. The capacity of the pump increased when the suction level was higher because the pump delivers the difference between the suction and discharge head. When the differential went down, the capacity increased.
  • Two pumps were installed in parallel, one cavitated the other did not. They had separate suction lines so that was not the problem.
    • Solution: Some one had installed an oversized section of pipe on the discharge side of the pump that was cavitating. The lower discharge resistance caused an increase in capacity, which caused the cavitation. When the proper sized pipe was installed the cavitation stopped.
  • The pump had been cavitating for some time, but after a visual check, everything appeared normal.
    • Solution: A globe valve had been substituted for a gate valve on the suction of the pump when maintenance found they were not able to get the gate valve they needed. A globe valve can add the equivalent of another 100 feet (30,5 meters) of pipe to the system.
  • The pump started to cavitate when a flange gasket was replaced on the suction side of the pump.
    • Solution: The inside diameter of the gasket was too small. It was acting as an orifice and restricting the flow.
  • The pump cavitated about one third of the time it was running.
    • Solution: A close inspection of the system revealed that there was no surge tank installed between the pump discharge and the multiple outlets that were using the product. The pump was acting like an accumulator and started to cavitate when the demand went up and the discharge head dropped.
  • The pump cavitated although there was excessive suction head available.
    • Solution: There was too much velocity on the suction side of the pump. This problem occurred in Norway in an application where the pump was taking a suction on a flow of water coming off of a mountain.

The seal was getting hot

  • The seal was showing evidence of running dry, but the fluid level was never lost in the pump.
    • Solution: Air was trapped in the stuffing box of a vertical pump after it was converted from packing to a mechanical seal. Most seal designs have no facility for venting the stuffing box in a vertical application.
    • Air can be removed from a horizontal stuffing box by drilling a vent hole through the end of the stuffing box into the void behind the impeller. Drill the hole at a 45 degree angle through the top.
  • The seal showed evidence of running dry. The tank had never emptied and the stuffing box had been vented properly.
    • Solution: The open impeller had been adjusted backwards and the “pump out vanes” on the rear of the impeller were pumping the stuffing box dry. This happens if you are using several brands of pumps and the maintenance mechanics confuse the impeller adjustment method. Some pumps adjust towards the volute (Goulds); some adjust towards the back plate (Duriron). It is easy to mix them up.
  • There was little to no fluid circulating between the two seals.
    • Solution: The pipefitting had bottomed out in a gland inlet elbow shutting off the flow. This sometimes happens after the seal has been repaired several times and the pipe thread shows some wear, letting it protrude further into the elbow fitting.
    • A cooling jacket was being used, but the seal continued to get hot. I have seen multiple reasons for this:
    • A discharge recirculation line had also been installed on the original installation, but it was hidden by some insulation. The cooling jacket could not keep up with the heat being added by the recirculation line.
    • The inside of the cooling jacket had become coated with a layer of calcium, because hard water was being used as the cooling medium. Condensate should have been substituted.
    • A thermal bushing had not been installed in the bottom of the stuffing box.
    • The cooling jacket flow changed with fluctuations in shop water pressure.
  • The dual seal was getting hot, but the mechanic had set the seal at the correct installation dimension. Everything appeared to be OK.
    • Solution: The inner seal of some double seal applications can get hot if the mechanic installs the cartridge seal by pushing on the seal gland and fails to reset the seal compression with the installation clips. The interference from the cartridge sleeve elastomer can cause enough resistance to compress the inner seal and unload the outer seal.
  • The seal faces got too hot because the mechanic had marked the seal location on the shaft sleeve before the impeller was installed. When the impeller was tightened against the shaft shoulder the sleeve moved and over-compressed the seal.
  • Most Durco pump impellers adjust to the pump back plate. When you make impeller adjustments you over compress the mechanical seal.

The seal would leak for no apparent reason

  • The cartridge seal had been hydrostatically tested with water and then put into a hot oil application. It leaked almost immediately.
    • Solution: The trapped water vaporized when the unit was started. This could be a dangerous condition because water trapped in a gasket and then flashed to steam could damage the equipment. Seals that are going into hot oil applications should never be hydrostatically tested with water. There are plenty of commercial fluids available for this testing that will not flash when exposed to hot oil.
  • The seal would start leaking about thirty minutes after the pump started.
    • Solution: The carbon insert would come loose in its holder when the seal came up to temperature. At shut down the metal holder would shrink and everything appeared normal.
  • The seal was tested in the shop, but leaked when it was installed in the pump that was operating at cryogenic (cold) temperature.
    • Solution: The faces had to be lapped at cryogenic temperature to keep them flat at the seal operating temperature.
  • The seal was found to be leaking every Monday morning.
    • Solution: A utility man did not know about seals. He would loosen the gland on the weekend so that what he thought was packing would drip a little. The regular maintenance people found the leak every Monday morning.
  • The leakage occurred during the winter months.
    • Solution: Someone circulated commercial anti freeze between two seals to act as a barrier fluid. The brand they selected contained a chemical to plug up radiator leaks and it kept plugging up the seal.
  • The seal would fail only during the winter months. The problem was traced to swelling of the dynamic O-ring but no logical reason could be found for its failure.
    • Solution: During the winter months a worker decided to oil the bed of his dump truck to make the, raw product slip off easier. The petroleum oil he used attacked the ethylene propylene (EPR) O-ring in the mechanical seal, installed downstream in the system.
  • The seal area was wet, but no visible leakage could be seen.
    • Solution: It turned out that there was a flange leaking above the pump and dripping the product next to the shaft.
  • The problem was traced to the fact that the mechanic was installing the seal at the wrong dimension. The written instructions were clear and placed in the box and yet the mechanic continued to do the installation incorrectly.
    • Solution: The mechanic could not read. He had been faking it for many years and was quite good at it. The same problem occurs with older mechanics that refuse to wear glasses, and as a result cannot see the funny little lines between the numbers on their measuring scale.
  • The outside seal in a double seal application failed suddenly. Nothing had changed in the system.
    • Solution: Routine maintenance included repainting the pump. The paint spray got into the outside seal springs and stopped them from moving.
  • The seal ran great for several days and then started to leak. It tested all right on the test bench after it had been removed from the pump.
    • Solution: It had been set screwed to a hardened sleeve and the set-screws gradually vibrated loose.
  • The seal was changed several times, but the steady leak persisted.
    • Solution: The leak was occurring between the pump sleeve and the shaft. This is a common problem in double-ended pumps that have been converted to a mechanical seal. You often have to devise a method of sealing the sleeve to the shaft or the sleeve to the impeller because the manufacture has not provided one.
  • The seal started to leak after many months of service. A bench vacuum test showed that the seal was all right.
    • Solution: The seal was fretting the shaft below the Teflon® wedge allowing the leak to come through this groove.
  • The seal ran approximately six months and then failed.
    • Solution: The lines were steam cleaned and the wrong grade of Viton® was in the seal. Most Viton® compounds will be attacked by steam, caustic or other water based solutions.
  • The seal was installed correctly, but it leaked immediately.
    • Solution: The solid, hard face is usually lapped on only one side. The face had been installed backwards and the rotary unit was running on a non-lapped surface.
  • The horizontal mixer would leak after twelve minutes. Not eleven, not thirteen, always twelve minutes.
    • Solution: The seal faces were opening because the equipment’s sliding foot had been bolted to the floor allowing the shaft to grow through the stuffing box when the unit came up to temperature. It took twelve minutes to come up to temperature.
  • The leak occurred because the pump open impeller was being adjusted without resetting the seal. Many operators make their own impeller adjustments and do not know that the seal face load must be re-set.
  • The centrifugal pump discharge was connected to the bottom of a surge tank. As the tank filled, the pump operating point shifted from too much capacity to too much head, deflecting the shaft in two directions.

Seal component damage is visible, but what is the cause?

  • It looked like a seal part had come loose in the stuffing box, but all of the parts were there.
    • Solution: During a previous installation a small spring had been lost when it fell into a drain hole in the bottom of the seal gland. It came loose after a later installation. This is a problem when several people work on the same pump.
  • The bellows plates were breaking but there was no evidence of corrosion, excessive wear, or vibration.
    • Solution: A discharge recirculation line was directing high velocity abrasive particles at the thin metal section of a metal bellow seal.
  • The inner seal of a double, rotating “back to back” design was showing excessive face wear in a short period of time.
    • Solution: The inner seal stationary face was not locked into the bottom of the stuffing box and when the system pressure overcame the barrier fluid pressure, the stationary face was pushed into the inside rotating face. When the pump was stopped the spring pushed the stationary face back to its normal position.
  • The carbon showed massive damage in a cryogenic (cold) application.
    • Solution: The carbon face had been lubricated at assembly and the lubricating oil froze in the cryogenic atmosphere.
  • The bellows plates of a mechanical seal showed massive wear on their outside diameter.
    • Solution: The seal was rotating in an abrasive slurry. Metal bellows seals should be designed to rotate the fluid inside of the stuffing box, instead of rotating through the fluid.

Other problems worth mentioning

  • The pump had been recently overhauled and at start up the pump was reading high amperage, but low flow.
    • Solution: One of the wear rings had been left off of the suction side of the impeller and the fluid was recirculating to the pump suction.
  • The pump made a terrific racket during start up. It produced the proper head, but the capacity was less than anticipated.
    • Solution: It was a two-speed pump and the second speed had been wired backwards.
  • In an acid application, a stationary seal showed localized corrosion only on the gland.
    • Solution: This was an older pump with a bolted on stuffing box that would slip because the bolts were worn. This caused the shaft to run against the gland causing it to overheat and, in an acid application, the corrosion rate of the acid doubles with an 18° F. (10° C) rise in temperature. It doesn’t make any difference if the acid or the part gets hot; the affect is the same.
  • The dual seal convection tank was running backwards.
    • Solution: The dual seal was not centered in its gland, and as the shaft turned, the close tolerance between the seal and the gland outlet increased the velocity of the liquid enough to drop the pressure and cause the tank to convect backwards.
  • The pump was converted from packing to a mechanical seal and then started to break shafts.
    • Solution: The pump was operating way off of its best efficiency point (BEP) causing major shaft deflection. The packing was acting as a bearing and supported the shaft during this deflection.
  • The product was solidifying in the stuffing box. Steam was being used to heat a jacket around the pump. The header gauge showed adequate pressure.
    • Solution: The gage was located too far away from the pump jacket. The line was not insulated and this allowed the steam to experience a pressure drop between the header and the stuffing box heating jacket. The result was that the steam cooled down below the necessary heating temperature. The problem was only visible when the pump was stopped for a period of time.
  • The nickel base tungsten carbide face was being chemically attacked.
    • Solution: A galvanic action occurred between the passivated stainless steel and the active nickel contained in the tungsten carbide face.

Posted

  • On February 18, 2018