Mechanical Seal selection 1

[maintennance]

I would like to know the basis for selecting a seal type. For example, when we specify seal flush plan 53B, on what basis the vendors select Face to Back , Face to Face or Back to Back . What are the factors to be considered for each type?
As a user , how to check whether the selection by the vendor is correct? Thanks to clarify.

 

[flexibox]

This is a very interesting question. Traditionally all double pressurized seals have been back to back. when designing these seals as a cartridge they required a top hat design and a relatively large stuffing box bore. There are many seal designs in the field arranged in this way. Some of the issues in this arrangement is the pumping ring design and circulation of cooled barrier fluid to the inboard seal. Although these issues can be overcome. These seals are externally pressurized. When API 682 was released it showed what appears to be a Tandem arrangement (face to face) but the seal in internally pressurised double seals. They are much more compact and fit the latest designed API682 standard seal chambers. They do have pressure limitations though unlike an externally pressurized seal.
To answer your question you need to consider the following:
Are you upgrading an existing API610 pump? If so then you may struggle to fit a new API682 seal in the chamber bore and dimension to nearest obstruction. You may have to replace the seal chamber housing to fit a new addition API 682 seal or modify the chamber to fit an old API610 double seal back to back with top hat. Or if this is a new pump application then the most likely arrangement you will see is a Tandem (face to face) as these pumps have standard API682 seal chamber dimensions. Seal companies have pre-qualified API682 seals.
1 If your pressure exceeds the pressure limitations of API682 catogory 1,2 & 3 seals then this will be an engineered seal and most likely default to back to back.
2 If your pressure falls within the pressure limits of API682 catogory 1,2 & 3 seals then you will most likely have a face to face seal.
Both arrangements work providing you follow the correct guide lines. Make sure that the seal design can circulate fluid to the inboard seal to remove heat and that it can be easily vented to remove any potential of trapped air.

trust this helps!

 

[JoeySoap]

True tandem = "face to back" seal configuration though?

 

[maintennance]

Thank U flexibox for your reply. Pl clarify as I am still not clear.

As per API 682 for pressurised seals (Arrangement 3) three face orientations are provided for contacting wet type seals viz 3CW-FB: the face to back or tandem arrangement due to the arrangement of seals in series, 3CW-BB : back to back and 3CW-FF: the face to face arrangement. Now in your explanation the tandem arrangement is mentioned as face to face which is in contradiction with the API designation and the literal meaning of tandem. From the fig 5 in API 682 3rd ed depicting the above pressurised seal arrangement it becomes clear that the basic difference between the tandem(FB) and other two arrangement FF or BB is that there is no provision for primary seal flushing in case of FF or BB arrangement.

Moreover if my understanding is correct in case of failure of outer seal in the BB or FF arrangement it will lead to loss of barrier fluid pressure loss and the inner seal can get blown open due to reverse pressurisation.This possibility is further aggravated in FF & BB arrangement as the inner seal is exposed to seal chamber pr inside increasing the opening forces in case of loss of barrier pr. In the FB (tandem) arrangement this situation is avoided as each seal can act as independent seals and ensure containment in case of failure of one of the seals. Further in the FB arrangement the seal chamber pr is acting outside the seal and hence the face will not get blown open. Moreover in FB arrangement the provision of primary seal cooling can improve the seal life and performance of the primary seal. Thus the FB arrangement is having significant advantage over the FF and BB and shall be the preferred arrangement. Pl confirm whether this understanding is correct.

Now coming back to the original issue , it is a new API pump with Plan 11 & 53B with tandem arrangement. At the purchase order stage this was agreed but during the execution stage the seal vendor is saying that the tandem seal will require additional free space for installation (due to the higher over all length of seal in tandem arrangement) than the specified minimum length mentioned in the API 610 which the pump manufacturer is complying. As an alternative the seal vendor is suggesting a double seal of face to face arrangement 3CW-FF which is shorter in length compared to the tandem seal. Now the question is whether this can be accepted or will it have any disadvantages compared to the originally agreed tandem seal.

 

[flexibox]

My apology that was a typing error. Agree FB is correct for Tandem. You are correct there is no provision shown for primary flushing in case of FF & BB arrangement however this can be arranged if absolutely necessary and it would be drilled into the stuffing box.
In the event of a seal failure of the outboard seal it will leak and the inboard seal may see reverse pressure however seals can be designed to handle reverse pressure. In fact most seal designs in FB,FF & BB are designed this way. You may have a retaining clip on the stationary face or it can be done hydraulically by shifting the balance diameter once reverse pressure occurs.
Primary seal cooling can improve seal life but the key is not to vapourize the barrier fluid at the seal face and provide good circulation as Plan 53B has a cooling loop and ensure proper venting is carried out. Most double seals only have plan 53B and do not require both (ie Plan 11 + 53B) this depends on the application whether it is going to be advantagous.
If the pump seal chamber dimensions comply to API682 then the Tandem seal arrangement should fit if the pump is built to API 610 then this may be why they cannot fit a FB seal. Designed correctly a FF seal should be fine but if I had a choice I would choose BB if it fits. (This is personal preference)
The key is not to get to reverse pressure stage and this is why Plan 53B is such a good system. Ideally you will have an operating pressure with "X" working volume, as seal leaks a low pressure alarm will alert operator to top up the system or it may be automated. An LL pressure trip set slightly above chamber pressure prevents reverse pressurization. Normally I select volumes in accummulator based on 28 days top up frequency and 2 ltr safety volume. So review the estimated seal leakage rates, Working pressure vs working volume and safety volume before seal enters into reverse pressure.

trust this is helpful.