Donald Phillips, engineer
Phillips Engineering, Melbourne, Fla.
DO THE ANALYSIS
There are numerous problems here: 1) unknown condensate pressure; 2) possible boiler-water corrosion issues; 3) an indication of capacity overload; 4) possible two-phase flow in the 6-in. vent header; 5) steam at multiple pressures added to a steam header; and 6) un-insulated pipe where condensation can occur.
The best solution to this problem is to continue walking down the pipe; get very familiar with the isometrics. Typically, these drawings are a mess. Create accurate drawings. Collect pressure and temperature data. Use an infrared gun to verify which traps aren't working. Identify the sources of the steam, which should give you the pressures. Measure the pressure at the suction side of the condensate pump. Re-calculate the capacity of the flash drum and discuss your findings with one of the steam equipment companies.
Don't forget to get samples of the steam condensate. There may be some problems with the steam-boiler feed water or some contaminant that's entering the return system from a heat exchanger leak. The resulting fouling could exacerbate the water hammer.
After collecting field data, run a heat-and-energy balance and hydraulic simulation of the process, including the pump. If you can't get exact flows, try using information from data sheets and, in desperation, vendor cut sheets. However, both the data sheets and the cut sheets can mislead you for a number of reasons but mostly because equipment seldom is operated at the design capacity. Recently, I compared field flow data averaged over five years with the nameplate capacity of a heat exchanger: 105,000 pounds per hour (pph) compared to 180,000 pph for the nameplate — that's 42% lower and only 58% of capacity. Now, imagine assuming the nameplate pressure drop of 4 psig for the shell. By ratio, the actual drop would be only 1.3 psig!
The simulation likely will indicate a need for knock-out drums in strategic locations, especially at the connection points between the main header and the previously unknown additions to the header. Drums will reduce the two-phase flow that accounts for the water hammer; don't completely re-insulate until you've photographed and inspected the pipe and of course, included it in your new isometrics. Adding more drums will mean additional steam condensate return pumps. Don't expect to get a lot of financial or maintenance support for this kind of project. Steam systems are supposed to run "maintenance-free" — utility maintenance can often be described as run-to-failure.
As for what's wrong with the transmitters, my first suggestion would be to check the grounding. The instruments at least should be grounded to a common ground so the analog signals roughly match. Of course, two transmitters never agree completely, so you'll have to see if you can live with this error or consider a new idea: replace them with a differential transmitter. Reducing the two-phase flow also may improve meter behavior.
Dirk Willard, process engineer
Superior Engineering, Hammond, Ind.
We make an anionic surfactant by reacting oleum (SO3 in H2SO4) with alkyl benzene (C12H25C6H5) in a continuous stirred-tank reactor (CSTR); see Figure 1. The product from our spray dryer is 85% pure, with the remainder being sodium sulfate and water. Ever since we started up this process a year ago, we've had problems: 1) a residue in the product layer in our separator; 2) a heavy black residue in the spent acid at the bottom of the separator — the company that regenerates our H2SO4 has threatened not to take our waste if we don't clean it up; 3) our titration analyzers downstream of the reactor and digester sometimes have fouled tubing — a sample pump failed a few weeks ago because a suction strainer plugged; and 4) startup and shutdown are rough — we can't turn down the capacity of the sulfonator cooler effectively and the smaller jacket control valve can't handle the mid-range cooling requirements. We see a lot of what appears to be burnt product during these periods. Recently, our Coriolis meter, which measures alkyl benzene, corroded out. What can we do to address these problems?
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