External circulation system for Plan 54 per API 614 1

[sharms]

We are implementing at our Refinery Plan 54 on pumps that handle heavy hydrocarbon products at high temperature. (Heavy Vacuum Gas Oil, Reduced Crude, Pitch, Tar, 260ºC-360ºC)

Based on the experience at other Refinery and the recommendations posted in this forum we have specified one console per pump.
To provide 3 years of uninterrupted service and alarms to detect operational problems we are considering an external circulation system per API 614, fourth edition.

Our seal provider proposes some difference respect to API and I would appreciate your comments.

1.- API specify the cooler and filters upstream the seal. It’s proposed to install them downstream the seal and downstream the pressure controlling valve.

2.- API includes an oil bypass line around the cooler with temperature control valve to regulate the oil supply temperature. The temperature control is not been considered.

3.- API requires a pressure regulating device to prevent pressure fluctuation in case both pumps are in operation. The pressure regulating device is not been considered. (On the discharge of each screw pump will be a Pressure Safety Valve (pop up action))

4.- API specifies an Pressure Differential Control Valve, PDCV, to maintain the barrier fluid pressure over seal chamber pressure. Seal providers only consider a back pressure control valve.

 

[flexibox]

I would suggest that you get the P&ID from your seal vendor and review their propsal with your Process & Instrument guys. You may find that Although it does not follow API recommendations, what they are offering is sound.
Just a few pointers:
1 If you are using Water coolers consider fouling over 3 years so take a close look at your cooling water quality and flow rates. Consider Natural convection air coolers.
2 Make sure you select the right barrier fluid. Royal Purple is as an excellent Barrier Fluid for mechancial seals.
3 The seals will most likely be High Temp metal bellows seals. Pay close attention to the seal arrangement specially if you have old API pumps.
4 Have you considered API Plan 53B using accumulators as a pressurizing system? very affective and reliable.
5 You may still need API Plan 62 (LP Steam or N2) on the hotter ones to prevent any potential coking on the external seal.

trust this helps

 

[sharms]

Thanks for your comments Flexibox.

Here some more details:

-Water coolers will be used. It’s also required that barrier fluid be at maximum 60ºC, so water fouling should not be a problem.

-Ok. It’s considered to uses Royal Purple 910.

-Seals will be metal bellows, face to back configuration (tandem) with rotating bellows per API Std 682. Some pumps are quite old (API Std 610 7th edition and earlier) but seal provider already verified available seal chamber size and mounting. They also confirmed that no retrofit is necessary.

-Plan 53B has been considered and discarded. As mention previously in this forum, in case of cooling failure high temperature could damage the blade accumulator. Plan 53B pumping ring would also not be capable to provide the required circulation to keep the temperature differential between barrier fluid inlet and outlet less that 16 ºC. Barrier fluid temperature would also be higher than 60ºC in normal operation.

-We have some pumps with Plan 52 and 62, but those pump operate with buffer fluid at high temperature (T>150ºC). As previously noted, it’s expected that in these pumps barrier fluid be less that 60ºC and therefore coking should not occur. Plan 62 is not considered.

Hope to know what other users are using as Console for Plan 54 on similar application.

 

[longeron]

Often times you'll find that the system designer has used an accumulator, like the one from a 53B, to provide back-up pressure in case the pump or pumps on the skid go down or the power source for the pumps goes down. No circulation would occur when the accumulator becomes engaged so there would not be a concern about heat damage to the elastomer.

You mentioned concern about a pressure regulating device. It may be worth it to consider some sort of control valve downstream of the seals to control the backpressure of the system using the pump spillback or another upstream control valve to spill back extra energy from the system to the reservoir.

You're lucky to have been able to get 1 skid per pump. I hate having to deal with systems with multiple seals supported by one skid. It makes it very difficult to troubleshoot.

Your 3rd consideration is something I've not come across, but I do like the idea. Being able to track the pressure of the seal chamber and adjust the barrier fluid pressure is worthwile in my opinion. At start-up the differential pressure between the barrier fluid and the atmosphere may be of concern- you must have the skid in operation before the pump is inventoried. Talk to your vendor about this and make sure that start-up isn't more difficult on your seal than necessary.

 

[bk19702]

Having just applied some high temperature Plan 54 systems these questions sound familiar.

The refinery I am currently onsite at originally wanted a 614 compliant system to supply 4 seals (2 between bearings pumps). Due to cost and the added complexity of troubleshooting, a "non-614" system was applied, with one skid per pump (2 seals supported). We utilized a temperature control valve on the system, along with an air cooled heat exchanger instead of a water cooled design. This kept the cost and subsequent cooling water requirements down. The cooling water system was already strained in this area of the plant, so this was an obvious concern. The air cooler will remove roughly 42,000 Btu/Hr, which were what the requirements for two dual seals called for. The air cooler has the added benefit of still providing some cooling of the barrier fluid even if the fan is lost for whatever reason.

I do like the differential pressure controller, but I have used the back pressure control valve in the past on similar systems and not had a problem. Regarding the barrier fluid, while I am a big Royal Purple fan, my company has found the 910 fluid has a coking tendancy. This was a result of some life testing that was done at our testing center. My personal preference is to use the GT-34 grade. This grade of fluid can be used in higher temperature services provided there is low residence time inside the mechanical seal, which there should be with a Plan 54 anyway. As you have already stated, you are more than likely outside of the realm of using a Plan 53, based on the required circulation rate to remove heat from the mechanical seal. In these services, 75% of the total seal heat load comes from heat soak, which will be significant at these higher temperatures.

Hope this helps.

 

[sharms]

Longeron,

An accumulator is also considered for the reason that you indicate. Seal vendor also recommend it to damp the pressure pulsation of the screw pumps. Those pulsations may cause damage to seal metal bellows.

Regarding to point 4, we will use a back pressure control valve and not a PDCV. Barrier pressure will be set above normal seal chamber pressure. We expect that there will be some abnormal situation (Closure of suction valve or wrong setting) where seal chamber pressure be higher than barrier pressure. Leakage will be detected through high level at the reservoir. In case of seal rupture a high high level a block valve will be closed on the return from the seal.

If the product reach the reservoir, an explosive mixture could be formed. Do other users have a nitrogen purge at the reservoir?

High differential pressure could be an issue for the inner seal at start up(high pressure at bellow inside diameter). Thanks for your comment; I will check it with our seal vendor.

Bk19702,

We are now implementing API 614 chapter 2 with some exceptions.

One of our pumps also is a between bearing pump. Did you use one control valve for both seals or one for each seal? Are there differences in seal chamber pressure?

Regarding to point 2, temperature control has been eliminated. Seal vendor assure barrier fluid can be operated at our minimum ambient temperature. I expect to see some minor temperature variation during night/day and summer/winter.

Although we will have a higher initial investment, we chose water cooler to avoid more noise and electric energy consumption.

Regarding to point 1, to maintain barrier side operating pressure higher than water side, cooler will be upstream the back pressure control valve. It’s still downstream the seal, but seal provider assure it would manage the heat in case seal rupture.

Best regards.

 

[bk19702]

The pumps in question have will have equal seal chamber pressures on either end, so it's relatively simple in that respect. Upstream of each seal is an orifice sized for the desired flow rate (equates to roughly half the total flow supplied by the system); downstream the seals are backpressured with the PCV.

 

[longeron]

I've never heard of N2 purge on the reservoir. I suppose it is possible. Most of the systems I know are not sealed from the atmosphere, they have breathers or are "splash proof" so the purge would easily leak out. I do know of a few that have oil mist connected to the reservoir cover to help keep contaminates out (pretty hard to get anything in the reservoirs that I've seen anyhow). I'm a little leary of this as I wouldn't want too much oil from the oil mist contaminating my barrier fluid.

If you are concerned about a catastrophic failure of the inboard or process seal you could add instrumentation to the skid that would alarm if the pressure in the line returning to the reservoir were over a certain point. It might be more advantageous to add a thermocouple to the return line and monitor the temperature of the barrier fluid. This could indicate that the barrier fluid flow to the seal is inadequate or could indicate process fluid intrusion. Some of the systems I'm famialier with also have a flow switch that is generally set for low flow- to indicate that the barrier fluid is blocked in. A high flow switch could also perform the function you require.

Talk this over with your vendor. I'm certain that they will have other answers to this question and will explain better than I can how that kind of failure has a very low rate of probability.