How to dimension the seal pot for a double seal in a installation?

[AngelM]

I want to know the things that I have to take into account to dimension the vessel for installing a barrier fluid in a double cartridge seal.
How to decide the dimension of the vessel pot, fluid to use, lenght of the line, controlers to install (fluid level, temperature,...) ?

Is there some guideline I can follow to make this kind of projects?

Thank you for your help.

 

[bk19702]

Consult the most current edition of API 682 for starters. There are general guidelines on NLL, distance from seal, instrumentation, etc., that can be referenced as a starting point. What liquid to use depends on the end user primarily - if you are using a pressurized barrier your barrier fluid will mix with the process stream and therefore it must be compatible. I would also suggest you work closely with your preferred mechanical seal supplier to determine the requirements of the seal design that must be met by the reservoir system.

 

[AngelM]

Thank you bk19702.
I have studied the API standard 682 and I have seen some considerations about the way of lubricating depending on the way of sealing an other things.

My question is: Imagine that we have already decided to use a double cartridge with chamber pressurized and I have to decide the dimensions of the barrier fluid vessel. I know that the fluid has to be compatible with the work fluid. we can imagine water.
The information I would like to have is how to decide the flow rate, the capacity of the vessel, the pressure to apply (+2bar??), dimensions of the connections, what we can know if we need a cooler or not (may be the work temeparture is not high but when working the fluid in the vessel pot can rise),....

Thanks

 

[bk19702]

If by flow rate you mean circulation rate required to cool the seal, that is going to be dictated by your shaft speed and the type of pumping ring (circulation device) installed in the mechanical seal. It's from there that the orientation of the reservoir comes into play. In general, with any closed loop circulation system for a mechanical seal, you want to have tubing with minimal restrictions such as 90 degree elbows, extra fittings, etc., while at the same time installing the piping in such a way that self venting of the seal chamber occurs. You don't want any sags, drops, or low points in your piping circuit for obvious reasons. Beyond that, I really can't give you any more detail without you providing detailed process information including temperature, pressure, speed, etc. Again, you really need to work with your local seal supplier as they will be best equipped to tell you what is required by the seal and how best to orient and design the support system.

 

[JJPellin]

I agree with bk19702. The seal vendor should be able to provide you with a recommendation for the size of the pot and the requirements for connecting it. We only allow 3/4" tubing with long radius bends. We do not allow any 90° fittings. We only use pots 5 gallons and larger even if the seal vendor suggests that a 3 gallon pot would be sufficient. A little extra capacity translates to a longer residence time and lower barrier fluid temperature. Larger tubing translates to a higher circulation rate which also allows for more heat removal. All of this contributes to improved reliability.

Johnny Pellin

 

[AngelM]

Thank you very much for your help.

I think the cartridge does not have pumping ring. When is it important to have it? What things have you to take into account in the cartridge connections and pipping if the pumping ring does not exist?

Is it better to install an additional cooler then?

Thanks

 

[bk19702]

How do you know it doesn't have a pumping ring? Have you consulted the seal drawing to verify? Do you know what the pumping ring looks like? My point is, if you are unsure about these things - get the seal vendor involved to verify. Trying to piece together information from the internet when you have not provided the complete picture as far as process information and conditions is a bad idea.

 

[AngelM]

I have seen some information regarding the pumping ring and its effect in the flow (specially with thermosiphon system(??).
I attach a drawing of a double cartridge seal back to back without pumping ring (I thing).
If there is not this devive and considering the position of the connections, do you think I could install a 53 lubrication Plan as a thermosiphon system o I should put a forced circulation fluid to improve the flow? And if due to the lack of the pumping ring the flow is not very good and the temperature in the seal pot increases, then to put a cooler?

What recommendations can you give me?

Thanks a lot.

 

[bk19702]

Doesn't appear to be a pumping ring installed from the drawing you have attached. I could not even begin to speculate on the cooling flow required by the seal since there is no information provided about the seal; no shaft speed, no process information, nothing. I seem to keep repeating this point. Who supplied the seal for your application? Ask them the required cooling flow - from there you will be able to determine if a Plan 53 will be sufficient based off shaft rotation or if forced lubrication is required.

 

[Airsporter1st]

The seal pictured has tangential inlet and outlet connections which, together with thermosyphon effects from a properly piped pot will promote barrier fluid circulation. Just be sure to get the right in and out connection depending on rotation direction.

Looking at the seal end view you have supplied - if the shaft is rotating clockwise then the left hand connection needs to be the outlet.

Use a minimum of 1/2" pipe or 3/4" tubing to connect between seal and pot, Mount the pot as close to the seal as you can and at least 18" or 500mm above the seal. Use all up-sloping pipe with the minimum of fittings and large radius bends.

The seal return to the pot is usually in the side and you should not allow the barrier fluid level to drop below the return line - or there is danger of nitrogen entrainment. Also thermosyphon will not work.

There should remain enough space above the liquid at full to at least accommodate thermal expansion. In practice, the upper (and lower) level of liquid is usually determined by a level gauge. The volume that the difference between high and low levels represents has to be compared with expected seal leakage and should be enough for 25 days between successive refills. So, if for example you have 2 litres between high and low level and your expected seal leakage is, say, 3 cc/hr, you will have 2000/3 = 666 hrs = around 28 days between top ups.

The previous posters are correct that by rights, you need to calculate the heat generated by the seal itself, plus any heat conducted from the pump casing and confirm that your seal pot is capable of dissipating that heat load. All seal vendors will have the ability to perform the heat load calculation and can tell you the expected temperatures in the system. As a general rule, with either water or oil-based barrier fluids you don't want this to exceed 80 degC (although may want considerably less on safety grounds - unless you are to put gaurding in place to prevent inadvertent contact with the system. Of course, if you are capable, it is not difficult to perform the heat load calcs yourself. (I've been in the seal industry 20 years, but I couldn't do it!!!)

Instrumentation wise, a Plan 53A vessel should have a level gauge, a low level alarm and pressure gauge and low pressure alarm. Barrier pressure should generally be set at 2 bar above seal chamber pressure.

A trap that many fall into is pressurising the pot with (typically) nitrogen and then shutting the nitrogen supply off - whereas in fact the nitrogen supply should remain on, so that you control only on barrier fluid level and not pressure.

All of the above info is available in API 682, which is a goldmine of information - especially the annexes and appendicies.

Hope this helps a little.

 

[AngelM]

Than you very much. Your information is very useful to me. Airsporter1st, I have revised the appendix in standard 682 regarding heat generation and het soak calculations and I have some doubts about calculating the amount of injection required. I don't understand at all how to calculate it for systems that use external devices to lubricate (API 21, 22, 32,...). How can I calculate the Qheatsoak?
I guess, in an normal situation one can calculate the heat genetation on the faces and once the flush fluid is decided and limiting the maximun temperature rise in the seal chamber (according to 682 standard) you can get the fluid flow. With this information one can now also if it is needed to have a forced circulation or not. Is it like this? All this is a bit confused to me.
For sure you have more experience in this kind of mountings.

 

[Airsporter1st]

As I said, I cannot do the calculations. I don't know enough of the fundamentals to use the formulae with any confidence.

What I can say is that, provided the pot is piped up correctly to the seal, you are unlikely to need forced circulation.

Good circulation is indicated by the difference in temperature between the return line to the vessel and the supply line to the seal. Ideally this should not exceed 10 degK, if it does then the circulation is poor.

You might however need additional cooling; either in the shape of a cooling coil within the pot or an external cooler. This is where you need the calculations - to determine whether the heat will be dissipated through the vessel alone or whether additional cooling is required.

It is the latter aspect which determines the overall temperature of the system.

To illustrate the above with some numbers:-

a) If, during normal operation, you had 52 degC on the supply line and 60 degC on the return line (measured, say, half way between the seal and the pot) then this indicates good circulation (8 degK difference)and adequate heat dissipation.

b) 85 degC supply and 90 degC return indicates good circulation but insufficient heat dissipation.

c) 40 degC supply and 60 degC return indicates poor circulation but adequate heat dissipation.

d) 80 degC supply and 120 degC return indicates both poor circulation and inadequate heat dissipation.

Hope this helps you to visualise things a little better. Sorry I can't help with the calculations.