API Seal Flush Plan 11 + 61

[CaracasEC]

Hi Guys,
Needing your expertise on seal plush plan 11 + 61 to be used on 98wt% Sulfuric Acid Unloading Pump. Original plan is 11 + 62 which is better. However because water or steam is not allowed (not recommendable) as a quenching media because water or condensate will react with the process fluid (sulfuric acid)original plan was changed. Is this reliable?
Pump Datasheet:
Service: 98wt% Sulfuric Acid Unloading Pump
Pump Operation: Intermittent Service
Normal/Rated Capacity: 15 / 16.5 m3/hr
Pumping Temperature: 19-40 deg C
Specific Gravity @ Pumping Temperature: 1.81
Head: 29.3m
BHP Rated:4.61kw
RPM:3600

Materials:
Case/Trim class: A-8 (API Class)
Case: SS316L
Impeller: SS316L

Mechanical Seal:
Seal Flush Pipe Plan : 11+ 62 (original)
11+61 (recommendation of the pump vendor because water or steam is not allowed (not recommendable) as a quenching media because water or condensate will react with the process fluid (sulfuric acid).

Thanks in advance and more power to this site...

 

[JJPellin]

A single seal in this service is probably not the best option. My first choice for this service would be a canned motor pump. I would also not recommend 316 SS. The temperature of the acid has a large affect on the rate of corrosion. I would only accept Alloy 20 or better. We installed a 316 SS pump in 98% sulfuric acid at 70 °F and had the case corrode through in 3 weeks. In our pump, there were areas within the pump where the temperature was much higher.

We have a dozen or more pumps in this service. All of them are Alloy 20. Several of them use a Plan 11 flush. But, I would want to convert them to gas seals or canned motor pumps. Any future pumps I purchase for this service will be canned motors.

Johnny Pellin

 

[CaracasEC]

Thanks Mr. JJpelin, the pumping temperature for this pump is between 19-40deg C. Is SS316L material not suitable? Will the seal last for seal plan 11+61. thanks...

 

[JJPellin]

I would not use 316 SS at any temperature in this stream. At 40 °C, I would expect unacceptable, high corrosion rates with 316 SS. I don't care for the risk of a single seal failure in this stream. I would choose a canned motor pump or a dry gas seal. However, it is possible to achieve good seal reliability with a single seal and an API Plan 11 seal flush in this service.

Johnny Pellin

 

[bk19702]

You need to make sure all the seal materials are compatible with the acid (faces, o-rings, metallurgy, etc). I would at minimum have the wetted metal components made from Alloy 20. The seal may leak, it may not leak; it depends on many factors. However, with a single seal, if you do suffer a failure you will be leaking acid to atmosphere, which is not a good thing.

 

[flexibox]

I have been involved in sealing this many years ago and the general selection for this concentration and temperature is 316SS wetted parts with Viton Orings. However a lot of Sulphuric Acid applications have externally mounted seals whereby they utilise a PTFE Bellows, in these arrangements there is no metal in contact with the process stream. They normally run dead ended Plan 02. See link attached

http://www.easterny.com/PTFE-bellows-seal-BMB-10.0.htm

In your case the what will happen long term is as the seal leaks crystals will form and the seal is likely to Hang Up. Is this an API Pump?

 

[CaracasEC]

Hi All, thank you very much for your informative comments.
@bk19702-All wetted metal components for the seal are made of alloy 20.
@flexibox-This is an API 610 Pump. For this application, will crystallization possible?
@JJpelin- I will push thru that pump will be manufactured with Alloy 20 materials (impeller, casing,) what about the shaft, should it also be an alloy 20?

 

[Airsporter1st]

As I recall from my high school physics, it is not a problem to add a small amount of acid to water, whereas it is not recommended to add a small amount of water to acid.

That being the case, if you are determined to stick to a single seal, then there is nothing wrong and much to gain in using water as a plan 62 quench medium - particularly in respect of avoiding crystallisation.

Ideally, you need a floating bushing or something similar outboard to contain the quench and thereby minimise water consumption. Don't forget also to pipe the water quench in at the bottom and out at the top (opposite to steam or nitrogen quench flow)to ensure that the atmospheric side of the sseal is fully wetted.