If all possible causes are exhausted, and the source appears to be inert barrier gas (gas entrainment) from the gas seal, a bleed line will have to be installed. System design and operating pressures will dictate the optimum destination for bleed off. Ideally, a small vent line (1/2 in. min) will typically be connected from the seal gland or the equipment stuffing box connection to one of the following:
- To the top of the suction tank; or
- To the final delivery point of the process fluid if it is at a lower pressure than the pump stuffing box.
In the event a bleed off line isnt installed, excess gas must be bled off the pump discharge (upstream of the discharge check valve) to a lower pressure location prior to pump start-up. The gas has the tendency to accumulate at the highest point in the system. Provided a bleed valve is installed, the gas can be bled through the bleed valve and shut off when fluid flow is evident. The pump can be started after bleeding is complete. The vent, when appropriate, may be used as a continuous vent or for recirculation.
Where a spare pump is on standby and under process pressure the barrier gas has the potential of pressurizing the spare pump. The amount of gas may exceed the pump capability. The same venting procedure, as above, should be followed.
Newer designs automatically track process pressure and maintain the optimum pressure differential between barrier gas pressure and stuffing box pressure which minimizes gas ingestion. Older designs have their gas set to handle maximum or worst case pressure, which at shutdown increases the likelihood of gas entrainment.
With gas seals being more compact and easier to use, more end-users are experiencing the positive benefits only gas seals can provide. Recently, a plant switched to a gas seal to minimize the overheating seen in a high pressure, bellows dual seal. The high pressure of the application produced a lot of seal face friction. At another plant, eight pumps were installed so they could have two in operation at any one time; the process fluid is a thermally-sensitive material that polymerized, causing the liquid seals to fail. A gas seal eliminated the hazards associated with of barrier fluid leakage.
Operators are enjoying the maintenance-free aspect of gas seals versus a dual-seal barrier fluid tank system. Recently a plant easily switched to gas seals to meet emissions requirements on light VOCs rather then adopt new dual seals and barrier fluid support systems which were the conventional choice in the past. A propylene booster pump had a single seal. The pump was a chronic poor performer due to the combination of low viscosity and high pressure. A change to a dual seal would require a high pressure, cooled barrier fluid system using synthetic oil. A gas seal was installed instead and has been operating for over a year with no cooling and problem-free. As users become increasingly familiar with the benefits of gas seals and proper application and selection, gas seals will continue to grow as the most efficient sealing device choice.
Scott Boyson is the global business development manager for Chesterton in Wakefield, Mass.; e-mail him at BoysonS@chesterton.com.