Mechanical seal or packing in VS4 pump?

[fe2655847]

Hi experts,

We have two VS4 type pumps mounted on a horizontal vessel with Ø2000 mm. One operating and one standby. Duty pump will start on high liquid level and will shutdown on low liquid level. If liquid level continues to rise to high high liquid level, the standby pump will start. Both pumps will shutdown on low liquid level.
Vessel operating conditions are as follows:
1. vented to atmosphere thus 1.013 bara (0 barg)
2. liquid handled is water @ 29°C
In the data sheet it was specified to use mechanical seal for containment of vessel vapors. However, with the operating conditions above vapor pressure of the liquid (which is water) at this temperature would be only 0.04 bara. Vapor is not likely to occur at this condition. Perhaps a packing would be enough instead of a mechanical seal? Per API 610, mechanical seals normally seal vapor; however, they shall be designed to operate in liquid in the event of tank or vessel overfilling. Please note that this vessel has an overflow nozzle installed 1812 mm above the vessel bottom surface so overfilling is not likely to occur.

Many thanks

 

[bimr]

A VS4 pump is a sump pump design, single stage, lineshaft pump covered by the API610 standard. In sump pump design, no mechanical seal or packing is needed for general service. In vapor proof services, mechanical seal is needed. Vapor proof or upper stuffing box seals are optional with a VS4.

http://gouldspumps.com/download_files/3171/b3171_CV3171.pdf

http://www.gouldspumps.com/download_files/3171/3171_Maximum_Sealing_Flexibility.stm

Packing will be adequate instead of a mechanical seal.

 

[JJPellin]

We have pumps of this type in oily water sewer sumps throughout the plant. They must be vapor tight and any vapors are directed through activated carbon canisters. Our pumps use greased packing for a vapor seal. We have similar pumps in other services that are equipped with dual lip seals with a cavity filled with grease between the two. We do not have any pumps of this type with mechanical seals. We have experienced one event involving a liquid leak from this group of pumps. The lower bushing assembly just above the impeller came loose in its fit and dropped down. This allowed it to block the leak off slots for fluid that leaked past the bushing on the back side of the impeller. With these slots blocked, the column flooded and liquid product leaked out at the surface. Following that event, we made modifications to all of these pumps. We now drill holed in the column about two feet below the mounting plate. If this were to happen again, the liquid would spill out of these holes and back into the pit. Other than that single event, we have had no problems with this type of pump using packing for a vapor seal.

Johnny Pellin

 

[TenPenny]

If the vessel is vented, there is little logic to sealing the pump. I would not go for the extra costs of a vapor proof design if the tank is vented to atmosphere.

 

[fe2655847]

The horizontal vessel collects water from hazardous open drains collection header. Does this has anything to do why the pump employs vapor proof design (mechanical seal)?

 

[bimr]

Of course. You originally said it was water, not wastewater.

 

[fe2655847]

I guess employing packing would be adequate because of the considerable gap of vapor pressure of water at 29°C to the atmospheric pressure.Or vapor proofing is optional per bimr's advise. Please note that the drum collects water from hazardous open drains collection header. Could be that they employed mechanical seals for vapor proof because the drum collects hazardous liquid?

 

[bimr]

You should keep the mechanical seals as per the data sheet. If you desire to change it, you should go back to the process engineer that designed it and put the question to the designer.