February's Process Puzzler: Improve a grandfathered unloading station

Readers suggest ways to improve a grandfathered unloading station in this month's Process Puzzler.

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Bob Driskell, engineer,
WorleyParsons, Texas City, Texas

Get rid of plug valves

Interestingly, I was an operations shift supervisor in a plant that consumed significant quantities (two to four truck loads/week) of concentrated H2SO4. We had similar problems.

Since this was one of my first jobs, I got to engineer some proposed solutions. We had a shorter line but a higher height to overcome to fill our tank. Our pump transfers were slow enough interrupt necessary operator functions. The first improvement was replacing our carbon steel/stainless steel plug valves with lined full-port ball valves on the unloading line to the pump.

Our plug valves required more force to operate, were very difficult to decontaminate after replacement, and occasionally leaked. The lined ball valves offered easier operation, better corrosion resistance, and weren’t contaminated (based on inspection of the first valve tested). None after that ever had any problem.

The socket welded piping was replaced with butt-weld piping and enlarged to match the 2 in. fittings on the trucks. Pressurizing the truck for transfer was never an option. I would maintain the sizes of the fittings from the truck to the pump suction but I would eliminate non-essential elbows.

We replaced the pump with another from the same manufacturer (a left over from another area) that was compatible in materials and flange-to-flange dimensions. Typical ANSI dimensioned pumps have very similar performance curves. They even have a radial vane-type impeller version (intended for high discharge pressures and low flows). A brief inspection of the pump curve showed that the replacement pump met our needs without affecting the electrical distribution or switchgear (same bucket, heaters, starter, and wire size). We used the radial type impellers for our reactor H2SO4 injection, very high pressure (relatively) and low flow. Careful selection of the pump and impeller should reduce NPSHR. As for the political situation, i.e., regulatory agencies, insurance companies, etc., we did not have any serious problems. However, that was many years ago.

We made improvements when we could. A PHA review on an anhydrous ammonia system recommended a requirement for transfer safety valves; these valves cut-off flow if a line ruptures. We appreciated the same potential for our other truck unloading stations and used it there. These transfer valves are closed without the need for an operator by a cable, directly or along the hose, clipped to the truck. A remote pull, or actuation is also usable. If they already connect and disconnect hoses for transfers, this device is just another valve, spatially speaking.

I cannot fathom any reason I would want to increase the charge pressure to the tanker-truck. There are too many unnecessary scenarios that result in potential injury for me to entertain that possibility.

Tim H. Gilder, production superintendent - VCM
Shintech, Plaquemine, La.

Raise the pad pressure

I believe the PCV set point can be increased up to 30 PSI, without any problems with the regulatory agencies. If required, a quick opening valve or PSV can be provided downstream of PCV in place of the rupture disc. A review will be necessary with the site safety manager and, perhaps, the trucking company safety manager. A PHA would certainly be necessary with an exploration into documentation of just why the set pressure was reduced. A fight over a pressure increase is probably worth it (raising the pressure will it contribute significantly to NPSHA); a typical PHA will take a week of meetings and preparations. The outcome may not be as expected — it may increase the price tag of your project.

The plug valves can be replaced with full bore ball valves, if this allows a larger pump and higher NPSHR. One concern I have is the material of construction of the new pipe; carbon steel is limited to 1 meter per second (m/s). Carbon steel is less expensive than stainless steel. Above 1 m/s, the iron sulfide layer protecting the pipe will be eroded.

Kamal Parikh, general manager of technical services
Reliance Industries Ltd., Surat, India

Split the flow

To increase the flow through the system, I would consider the following:

  1. Twin the suction pipe to reduce the head loss in this section;
  2. Although it will probably have a minimum effect, decrease the backpressure in the receiving tank;
  3. Install a new pump that will meet the new process conditions; and
  4. Increase the line size where possible to reduce the pressure drop.

Running a second suction line may make it easier to snake the pipe through the existing space. Another option might be to consider two pumps — in parallel; this could be difficult to control and lead to a few operations issues. Decreasing the backpressure will have a minimal effect because fans typically run at the 1.5-2 psig range. Increasing the fan draw will increase wastewater generation slightly unless the extra acid captured by the scrubber is recycled in the process.

Jim Darby, process design engineer
LANXESS Inc., Sarnia, Ontario

Avoid carbon steel

This isn’t a solution to your pumping problem, but more of a caution, be careful when increasing the velocity of sulfuric acid through piping. If carbon steel pipe is being used for the transfer of the sulfuric acid, the velocity of the sulfuric acid in the pipe must be kept low or it can cause corrosion problems. In carbon steel piping the sulfuric acid forms a sulfate layer on the pipe. This acts as a corrosion barrier in the pipe. At higher velocity, this barrier gets washed off and then it just keeps trying to reform again and again, corroding away the pipe. This corrosion can be very rapid and is especially apparent in elbows and other changes in direction. For your pumping problem, can the pump be lowered any (excavate and areas to mount the pump in) to provide more room and more NPSH for a higher capacity pump?

Bob Welsh, reliability engineer
Arkema, Inc., Carrollton, Ky.

April's Puzzler

During a routine walkthrough at a new job we found a problem with how the relief valves are laid out for a distillation column (Figure 2). After further thought, we wondered if the piping is undersized. A review of the relief scenario and a mechanical sketch of the piping reveal some background. The relief valves are sized for a total flow of 170,000 lb/hr for fire and other scenarios; fire isn’t the worst. All valves are conventional, not pilot valves or balanced bellows. Sizing was performed for a mixture in the column of 64% ethanol and 36% ethyl acetate. The relief conditions were: 150 psig and 149ºC. There are other problems with the set-up, which need to go to the maintenance manager right away. What should go in the report? What must be done to improve the safety of the relief system?

Figure 2. What is wrong with this pressure relief system?
Figure 2. What is wrong with this pressure relief system? (Click to enlarge.)

Send us your comments, suggestions or solutions for this question by March 16, 2007. We’ll include as many of them as possible in the April 2007 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.


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