Properly Sizing A Steam Trap – Part Two
My name is Kelly Paffel. I want to talk about steam trap sizing. This is part two of a two-part series, and I just wanted to do an overview. I’m the technical manager for Inveno Engineering LLC. We’re located in Florida, and we are a domestic international firm specifically for engineering, and steam, and condensate systems.
Sizing a steam trap properly is critical to ensure high reliability. When sizing a steam trap there are a number of items that need to be documented and the following example will review. One of the things we need to do is come up with a sizing factor. All steam trap manufacturers will have a sizing factor for their condensate capacity charts. For example, the inverted buckets sizing factor is three to one. What does that mean? If the steam or condensate capacity is 200 pounds per hour, and then take that number x three times 200 or 600 pounds per hour. So you want to select a steam trap for 600 pounds per hour in the capacity chart. For float and thermostatic steam traps are two to one sizing factor. Thermostatic is three to one sizing factor and thermodynamic is 3 to one sizing factor. We talked about par one is determined, the steam trap, discharge pressure, condensate return line pressure.
So the thing with this here is that we need to understand P4 and P3 in this system. So we get our differential pressure and differential pressure is really critical when we’re coming up and making the correct selection of the steam trap.
Evaluate the condensate flow condition for the steam trap operation. We have two types of steam traps, no matter what steam trap is manufactured there are two types. One is continuous flow and the other one’s on-off operation. So when I come up and say, Oh, I have a heat exchanger, got a modulating valve. That’s going to be a continuous flow to the process on my want to go with a continuous flow type steam trap or operational design. The other operational design is on-off. The other thing is to determine the orifice inside the steam trap. One of the things we need to know the maximum steam pressure or body rating of that steam trap.
So at P1 we need to know here, what is the maximum pressure that can come down or steam pressure can be applied to the system? So the safety valve is set for 150 PSI, the orifice inside the steam trap has to be rated for 150 PSI. The next thing is that also comes up with the body rating. So the body rating has to be rated for 150 PSI at 366 degrees because we’re underneath the code, the power piping code, or depending on what country there are codes that govern steam and condensate piping. The operating pressure and the example would be 75 PSI. Then we need to know the inlet to the steam trap and the minimum differential pressure. In part one we talked about how to come up with the pressure drop across the control valve, how to come with the pressure drop through the heat transfer. So we come up with P4 and P5, which is outlined in this presentation.
So, therefore, we select a maximum pressure rating, which we already said that the maximum pressure rating would be 150 PSI, 366 degrees. Then we come up with P1 minus P2, which our example, we came up with a differential pressure of 55 PSI. The condensate capacity sizing factor. We’ll just take 200 pounds per hour, we put a sizing factor to that, which is 600 pounds per hour. So on this site right here, as you can see here is the maximum orifice pressure rating. So we must find something that’s 150 PSI rated or higher.
So this is the differential pressure. We said the differential pressure is 50 PSI. So we come down this column, 50 PSI till we find something 600 pounds per hour. We come to the first one is 1400 pounds per hour here. Well 1400 pounds per hour, if I go over to the left-hand side, the orifice is only rated at 50 PSI. We said that we had to have the orifice rated for the maximum pressure, which is all 150 PSI. So if we come down 150 PSI, 50 PSI differential, and then the capacity is only 410 pounds per hour. So we had to come down to the next model steam trap where the 150 PSI and the capacity is now 990 pounds per hour. So, therefore, we come up with the correct steam trap selection.
Backpressure in the steam system, the backpressure is always going to be there in the steam system, which means that the thing with the steam pressure, the back pressure, it can be intentionally designed or because of the installation. A high percentage of the steam trap application, they are going to be pressure on the discharge side of the steam trap that is given. One of the biggest problems we have in the steam trap sizing is we don’t know the back pressure against the steam trap and probably 95% of the condensate systems. There is back pressure.
Backpressure became unintentional or deliberately produced by the design of the system because today we are looking at implementing pressurized return systems to increase the steam thermal cycle efficiency, which means there is going to be intentional back pressure at that system. The other thing is, that we take a rise here, up after the steam trap, for a rise in the piping system there’s going to be back pressure and a rule of thumb is a half a PSI for every foot rise in pressure. So therefore, there is going to be back pressure onto the steam trap or down here at P4. The other factor is that undersized condensate lines, which I already mentioned is a big factor today and steam and condensate systems. So therefore, 95% of the time, we’re going to have back pressure onto the system.
Now, as I said before, this is a condensate line pressure intentional that today we designed condensate systems to be pressurized so we can bring the condensate back to the boiler plant operation under pressure increasing the steam thermal cycle efficiency. Its really critical that you have pressure gauges on the condensate system so you understand what that pressure in the condensate systems going to be. Another example here is a drip leg steam trap, which is a common steam trap out there operation P1’s, 150 PSI, we have 25 PSI back pressure two PSI for the rise in the pipe, condensate pipe because it’s 10 feet rise. So we really have 30 PSI differential. My flow is 120 pounds per hour. So I go and the sizing factor is three so I’m going to use the thermostatic design steam trap. So three times my capacity, I have to pass 360 pounds per hour condensate, my differential is 125 PSI.
So if I come down to my sizing chart, the maximum pressure rating we said was 150 PSI. So all these steam traps here have a maximum pressure, 450 PSI. So it’s not as prevalent in the mechanical design steam traps. So I come here to 125 PSI differential find something greater than 360 pounds per hour. I see you have a steam trap here, 838 pounds per hour so that’d be a model 452, and then I made my selection of my steam trap.
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