Dual mechanical seals 2

[mfakharmechanica]

Dear Sir/Madam,

Would you please explain when dual mechanical seals are used? I konw fr the hazardous services (toid,...)dual mechanical seal are used.
We have some centrifugal API 610 pumps with hydrocarbon fluid (Aromatic Feed, Naphtha,…) with discharge pressure 13 barg. Some famous manufacturer like Sulzer, KSB… presented dual mechanical seal with plan 52 and 53B. There is not any available utility in our site for their consumption.
Some other manufacturers presented single mechanical seal! I don know whay dual mechanical seals are needed?

Thanks

 

[BigInch]

I refer you again to mcnalley institute. You need to read as much as you can of all information at this very good pump website. It has answers to all your basic questions about pumps.

Dual seals are recommended for a variety of purposes that include:

To prevent a costly product from leaking.
To prevent a dangerous product from leaking to the atmosphere.
To prevent a pollutant from escaping to the atmosphere.
As a back up seal to prevent costly down time when the first seal wears out or fails. This is an important element in any predictive maintenance program.

Find the rest here,

http://www.mcnallyinstitute.com/08-html/8-8.html

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mfakharmechanica]

Thanks Biginch,

The problem is that there arent any avilable utility at our site. The site will be new Tank farm and there are much process pumps to pumpig the hydrocarbin into the ships. As I mentoned there are not any source of air, cooling water,...for this meen and also manufacturers recommended dual mechanical seals for some pumps! Medium fluid of all pumps will be hydrocarbon and some pumps submitted with dual mechanical seals! Others are single!
Whay we should do for this?

Thanks agin,

 

[BigInch]

Then you will have to provide a certain minimum functionality (perhaps via temporary utilities, including electric generator, lighting, fire protection, temporary oil water drain and sump tank, hydraulics or neumatics for ESD valves, security lighting, meter operation, minimum communications to ship and operators, etc., in order to safely start and operate your pump station in "test" mode, if it must be started up before permanent facilities are installed. Without all necessary utilities and safety equipment, don't do it.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mfakharmechanica]

Thanks.

 

[longeron]

If we're talking about dual pressurized seals the utility requirements can be quite manageable.

API Plan 53 refers to the support systems for dual seals.
53A is the most common installation- a Nitrogen header used to pressurize a seal reservoir.

53B is less common and utilizes an accumulator to provide pressure for the barrier fluid between the seals. This could be an option for a tank farm provided that the end user has adequate instrumentation to notify operations or maintenance that the barrier fluid needs to be topped off, or that the accumulator bladder needs to be refreshed.

53C is even less common and uses a piston pot tied into the discharge or seal chamber of the pump to provide a reference pressure for the pot. Again, instrumentation would be required.

Cooling for these plans may also be an important consideration. If cooling water or other fluid is unavailable (as mentioned above) an air cooled heat exchanger may also be used to cool the barrier fluid. When specifying an air cooled exchanger your local environment is an important factor. Salty air or high humidity for example may need to be dealt with.

Your best off discussing each application with your preferred mechanical seal vendor. They may have other options or can further discuss the reasons for the multiple seals in your services.

 

[BigInch]

Artisi, No worries.

http://www.mcnallyinstitute.com/04-html/4-6.html

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[scalleke]

So can someone tell me why a double mechanical seal is required here?

Hydrocarbons in a tank farm?

Use a single seal and vent the vaporised leakage off to the top (vapour section) of the tanks.

To reduce the leakage a tandem seal could be used. Just connect the cavity between the first and the second seal to the suction side via an orrifice plate. That way you will be sealing on suction pressure and not seal cavity pressure. That will reduce the leakage.

Traditional hydrocarbons should vaporise off over the seal face so hooking up to the vapor side of the tanks should not be an issue.

By the way a double seal does not provide a backup seal as only one seal is ever sealing to the outside. Tandem seals do provide a backup seal

A double seal is used for one reason. To make sure that absolutely no pumped product leaks to the outside.

Regards
Scalleke

 

[BigInch]

As you yourself have said, "To make sure that absolutely no pumped product leaks to the outside."

http://www.hse.gov.uk/foi/internalops/hid/din/521.pdf

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[longeron]

Many end-users utilize dual unpressurized, tandem, seals in tank farm applications to provide measure of back-up. I was surprised when I first saw this as well. It makes some sense, as tank farms can be expansive and fairly un-manned areas. Some end-users are now applying VOC repair rules to every application- any drip, or evidence of leakage. On top of that, many of the insurance companies now require risk analyses of applications that can force some of what we used to think of as low priority applications into the forefront.

Sometimes this can make applications more difficult- like crude oil. This is an easy application for a single seal. In my experience dual unpressurized seals have a more difficult time with this fluid due to heat generation and high viscosity. Crude oil in the buffer fluid does no favors. Again, your prefered vendor's experience and knowledge of their product line is invaluable.

 

[BigInch]

Low priority? Not tank farms. Typically in and around tank farms, aside from the relatively few mainline pumps at stations out on a pipeline, is where you will find most all of the pumps, hence, most of the vapors, drips, leaks, large volumes of stored products, ships, trucks, sparks, loading racks, injection pumps, flanges, valves opening & closing, product sampling, start-stop ops, operation personal, as well as all the risk factors and accidents that seem to happen in conjuction with those features too. The only good thing about tank farms is most pressures tend to be lower than mainline ranges, but that doesn't seem to make much difference.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[gyaneshwarp]

Where ever you need a back up for the process fluid, dual seal is recommended.
For satisfactory performance, the liquid in the seal area should remain in liquid state all the time. To ensure this whatever minimum utilities are required, please use the same. Do not go for unnecessary factor of safety.