Question from December's Chemical Processing
A trucking company that supplies our fuming sulfuric acid has offered us a discount in price if we can double the pumping rate of our transfer pumps (Figure 1). Management is excited about saving money but an inspection of the unloading station doesn’t inspire confidence. A new pump may be purchased but the suction line is limited because of the suction piping; spacing near the truck is very limited. The plug valves used throughout pose a significant pressure drop. Compressed air is already being used but is regulated down to 25 psig because of the danger of fatigue on the rupture discs protecting the tanker truck (35 psig is allowed by code; the setpoint for disc is 50 psig). What can be done safely to increase the transfer rate? Can we double the rate? Are there any foreseeable problems if we do increase the rate? What improvements are needed in this old, grandfathered unloading station?
Increase the NPSHA
To increase the rate of the fuming sulfuric acid unloading the following steps should be taken:
- Determine the benefits, the net amount per year that could potentially be saved by doubling the rate.
- Calculate what flow this new rate would need to be.
- Perform the hydraulics on the entire system from tanker into the storage tank — pay special attention to the suction line eliminating bottlenecks. Do the same for the pump discharge line. The problem stated that plug valves are a significant pressure loss. Look into replacing them with full port plug; you could consider full port ball valves or gate valves as they are less expensive and/or eliminate some of them if possible.
- While doing step No. 3, evaluate the existing pump first to see if it can be modified to meet the new flow requirements and head requirements of the system.
- If the pump must be replaced, what are the changes? Is a larger impeller or motor needed? Will the pump base need to be modified (obviously, the concrete needs to be repaired in the dike and the base). You need to establish the basis for design to meet the new operational requirements.
- Next, you need to determine if there are new regulatory provisions that have cropped up for this grandfathered system. Seal requirements have changed over the years. Be careful to match seals to new American Petroleum Institute (API) standards for minimizing fugitive emissions! There might be Department of Transportation (DOT) regulations that also call for certain things such as auto shutoff break-away quick connects. I'm not sure if it's required by DOT or some other agency but there are “no-drip” or dry quick connections that shut off valves in case of a break. I'm not exactly sure with trucks but I know for rail cars there is a state rail DOT inspector who shows up at our site a couple of times per year and there are DOT requirements that our rail unloading system has to meet.
- There may be some improvements in design to include. Carefully consider impeller design and other pump details.
- Consult with production and find a design that won't deliver on performance at the expense of reliability. There may be some historical records steering you away from some of the options you have considered thus far.
- Finalize your pump specifications and talk to vendors. Be wary of the quotes. Vendors like to cut the pump size and impeller trim to a minimum because of lowest price. Low clearance may contribute to corrosion in ways you can't imagine so try to duplicate conditions in the old pump, (if it was reliable) like tip speed, or such.
- Confirm your electrical system. The starter is probably okay but the wiring and conduit may have to be replaced — especially if they’ve seen service around acid. All of this could add to your project cost.
- Get quotes or estimates for any piping changes, see site maintenance, mechanical contractor or engineering. You can’t really prepare final drawings until you specify the major equipment.
- Evaluate the vent system on the tank. Is that scrubber capable of handling the increased venting rate? Are the vent lines large enough to not put too much back-pressure on the fuming sulfuric tank? Will the operating pressure still be significantly below the tank relief set pressure? You should probably do this separately from the HAZOP review required for the project. Once you have completed this relief device contingency evaluation the costs should firm up. If there are any modifications required for the tank vent system you need to define the cost.
- After collecting all this information and distilling it into a total project cost and scope you need to do a cost/ benefit analysis, e.g.., payback, rate-of-return.
If the project makes economic sense the next step will to take a closer look at the regulatory requirements. You may need to upgrade your permits. Assuming that nothing serious comes up, then you need to do a HAZOP which will probably affect your project budget and require a repeat of Step 13. Talk about trying to unload a truck a little quicker, on the surface it didn't seem that complicated.
George J Bushey, production engineer
GE Advanced Materials, Selkirk, N.Y.
Pump in a sump
Trying to please management in this situation will be very challenging. For what it’s worth here’s my two cents: put the new pump in a pit by the truck; increase the suction line size and tie the pump output to the old suction line. Replace the plug valves with ball valves if applicable to the service. Another thought is to run a second “same size” suction line parallel to the first but that wouldn't solve the bottleneck at each end. The pressure in the tank could be raised, but the differential is probably already at “critical” and the net increase in flow would be small if any at all. All of these ideas deserve a review. The final proposal won’t be elegant and will create some other problems, but the project should be considered since the concrete must be replaced anyway. There will be some concerns about the sump leaking into the ground. The process won’t be grandfathered in anymore, so no doubt there will be other improvements to be included in the price. Once all this is done, the economic incentive must be evaluated.