When coking can happen in mechanical seal 3

[Sam993]

Hello All:

I would like to know what is the mechanism of hydrocarbon coking in the atmospheric side of the mechanical seal. Does it happen only in hot hydrocarbon? Do we need to use steam quench for low temperature hydrocarbon in order to minimize coking?

Best Regards,

 

[flexibox]

Coking normally occurs on a hot applications and it is recommended to use API Plan 62 (LP Steam) Sometimes steam is not available and if this is the case then you may use Nitrogen. Coking does generally occur with high temperature Hydrocarbons however there are other products that crstalise (as opposed to Coke) on the atmospheric side of the seal that also require API Plan 62 but steam may not be the right quenching medium. For example caustic will crystalize but you would not use steam as a quench but Water. If you use steam you will heat the seal face and potentially vapourize the fluid film. With low temperature hydrocarbons you may experience icing. In this case even when Dual seals are used Plan 62 (LP N2)is sometimes used to prevent icing on the atmospheric side of the seal becuase the application may be so cold that the whole pump bowl ices up including the back of the seal so the N2 prevents the icing at the seal.

trust this helps

 

[bk19702]

Please also remember that a steam quench on hot services is a preventative measure against coking; it will not remove the coke once it has formed. Interupted periods of quench to the seal in these services will most likely result in coking and potentially shortened seal life. In my travels, this is one point I stress to operators with regards to maintaining the quench on coke-prone services.

 

[sharms]

Hydrocarbon coking is a thermal conversion of heavy oil to lighter products and petroleum coke, a solid product. As hydrocarbon leak through the hot seal faces it takes temperature and coke could be formed.

Steam quench (Plan 62) is specified to cool the atmospheric leakage and prevent coke formation when temperature is above 177ºC or known coking service.

Probably some coke will be formed on services at lower temperature as the hydrocarbon pass through the hot seal faces. Steam would help to cool seal leakage on the atmospheric side but would also heat up the process side and make the situation worse. Therefore I would not recommend steam quench for the solely reason to minimize coke formation if the process side is less than 150ºC.

In the past, steam quench has also been used with single seal and viscous hydrocarbon (such as asphalt), to keep the products thin and avoid damage at start up. Now a day, due security concern, we would think twice before using just a single seal for high temperature service.

Best regards

 

[bingopin]

The purpose of the quench is to keep the hot vapors from oxidizing. We usually use low pressure steam but sometimes it's difficult to keep the steam "dry" enough. On critical services, we use a nitrogen quench.

 

[gyaneshwarp]

Coking normally takes place at higher temperature.
It may take place even at lower temperature if heat generated in Mechanical Seal is not removed properly.

My recommendation would be to go for API Plan 21 , ensuring flushing temperature below 120 deg C. By doing this you may be able to get rid of quenching.

 

[bk19702]

You need to be really careful before randomly applying cooled flush plans to single seals in hot, heavy, hydrocarbon services, which is where the steam quench is ideally suited for use. Not only will cooling a heavy, coke prone process possibly result in a fouled heat exchanger and low flush flow to the seal, it will also place quit a load on your heat exchanger. You need to make sure you've really taken into account the heat load of the entire circuit (seal generated heat + heat soak) if you are going to try and apply a Plan 21 or similar. These are not 'plug and play' systems and really require a good bit of engineering analysis up front. Cooled flush plans like Plan 21 or 23 are ideally suited for hot water and HC applications close to their vapor point. Hot, heavy, hydrocarbon streams are better suited with a Plan 62 as a secondary flush plan to avoid coking, or a dual pressurized seal system instead for added reliability / safety.

 

[rzrbk]

Bingopin brings up a valuable point. Cooling the seal faces with steam quench is not always necessary, and wet steam will do more harm than good.

A nitrogen quench will not cool as well as steam, but will keep oxygen away from the faces to prevent coking.

 

[longeron]

As discussed above, wet steam can cause serious damage to a seal.

Imagine a slug of water in contact with the hot seal faces- sudden vaporization of the water could be bad- physical shock, and sudden contact with cold water could be bad- thermal shock. Both of these could cause a brittle seal face to fracture.

Steam traps near the seal will help, or an old solution is to wrap the steam line around the discharge of a hot pump.