THIS MONTH'S PUZZLER
We are experiencing water hammer and failure at low-point elbows in our steam-condensate return system. Several old pipe supports have failed. This has become a particular problem in the past few years. The original system ran well with few headaches. The 6-in. schedule-80 pipe carries 75-psig steam condensate back to an old flash tank through a 500-ft pipe. On inspection, I discovered that much of the insulation has been removed and that the header receives condensate from several additional lines, including three 6-in. bare lines running 400 ft, 300 ft and 150 ft. I'm trying to identify the pressure of these condensate lines. It could be 150 psig; the refinery has 400-psig, 150-psig, 75-psig and 25-psig steam. The steam traps, which seem well maintained, are noisy when equipment is running. The condensate return pump at the flash tank is showing signs of wear; it runs nearly continuously and has needed more maintenance than previously. In addition, the backpressure is maintained on the steam return by two pressure transmitters (on the inlet and outlet of the control valve) that wander all over when operating; the operators leave the control valve cocked partially open without bothering with it. What can be done to save this old system? What's wrong with the transmitters?
INSULATE THE LINES
Water hammer is the force generated from a slug of rapidly moving water travelling at the steam velocity that is suddenly stopped. A rapid change in phase, caused by pressure reduction, can create this phenomenon, converting the flow regime from all liquid to slug flow. I am assuming the flash drum runs at 25 psig, the lowest pressure mentioned, because this is the only way all sources you have listed can get into this flash drum. A slower phase change is caused by pipe heat loss.
Insulate the lines. Keep in mind that if the lines are not insulated, the pipe is acting like a heat exchanger. You are creating conditions that can cause potential unstable flow regimes, i.e., slug flow due to excessive cooling, pressure drop due to cooling and the potential of condensing steam back into water under the right circumstances. Insulating the lines creates a uniform pressure and heat transfer profile through the pipes that can help stabilize the flow regime. Having these lines connected can create unstable flow regimes in other sections of the lines as well.
Pipe fittings are extremely susceptible to unstable flow regimes such as slug flow. As a slug of water moves down the pipe, pipe fittings (elbows), control valves and instrumentation can be damaged when this slug is suddenly stopped. The force is tremendous. This is why your pressure transmitters are not functioning properly. In an elbow at the point of highest velocity, you also have the lowest pressure. Because the fluid is saturated, a loss of pressure will flash water into steam and this, combined with the force of stopping the slug of water, is eroding the elbows. When you pull the elbow and inspect it, you should see pits at the bend. Insulating the pipe should help stabilize the flow regime and minimizes the rate of heat transfer that is occurring between the inside fluid and the outside air. Where possible, replace 90° elbows with 45° elbows. This will reduce velocity due to the reduced resistance factor of a 45° elbow compared to a 90° elbow and may improve mechanical reliability. The straighter the pipe run, the better the ability to handle an unstable flow regime because the slugs or waves have nothing to stop them.
Piping has limited space to absorb impacts of the forces generated due to these unstable flow regimes. Connecting steam lines of different pressures together will make water hammer worse because the connection points are where the slugs are stopping and forces are exerted. Instead of tying lines into other lines, try to relocate the lines closer to the flash drum or into the drum, if possible, to help mitigate the forces exerted on the pipe and the system. Make sure you insulate the lines effectively around the drum.
With regards to your pumps, make sure you have adequate net positive suction head available (NPSHA) for your pumps to function without cavitating. Check the liquid level in your flash drum to be sure your suction is flooded, if this is a horizontal pump. The shock wave resulting from the sudden stopping of the water slugs could possibly get far down the lines to impact the pumps. In addition, keep in mind the un-insulated lines could also be delivering cool condensate to the flash drum and cooling the flash steam in the drum rapidly. This could result in forces that could be impacting the pumps and damaging them.
• insulate all lines — I suggest mineral wool;
• relocate lines closer to the flash drum, if possible;
• replace 90° elbows with 45° elbows, if possible; and
• check and validate the NPSHA for pumps to be sure you have enough.
Finally, if you desire to know the amount of flash steam being generated, start at the beginning of the 500-ft run and add in the other sources once you determine what the actual source pressure is at each take-off — the individual runs of 400 ft, 300 ft and 150 ft, respectively. Determine the pressure drop from each run based upon the amount of condensate contribution from each source and its source pressure and temperature. The amount of flash is a function of ƒ (saturation temperature, drum pressure, source pressure and temperature). This can be found using the steam tables, a Mollier diagram or simulation software.
Eric M. Roy, principal engineer
Westlake Chemical, Sulfur, La.
SEPARATE THE CONDENSATE
Install a flash tank upstream of the control valve to trap the condensate. Pad the drum with compressed air and regulate the pressure. Feed the condensate into the bottom of the drum. The condensate can be recovered for the boiler house. The control valve will see only steam.
THIS MONTH'S PUZZLER