Differential pressure transmitter issues with steam metering and condensate water
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I ran into a problem today and am seeking advice from experts: The device in question is an orifice plate flow meter used for measuring steam. After draining the waste fluid, after some time the pressure tap no longer felt hot to the touch, so I started using it again! The length from the differential pressure transmitter to the condensate ball pressure lead pipe is 4.5 meters, and the water column height is 1.5 meters. Question 1: The three-valve assembly is fully closed. Following the procedure for bringing the three-valve assembly into operation, I first opened the balance valve, then the positive pressure valve; I also vented the air from the positive and negative chambers of the transmitter. After that, I closed the balance valve and finally opened the negative pressure valve. At this point, the transmitter is ready for use. I’m not sure if this procedure is correct If it’s incorrect, please feel free to correct me! Question 2: The three-valve assembly is fully closed. The current sequence for operating the valves in this assembly is to first open the balance valve, and then open both the positive pressure and negative pressure valves simultaneously; at this point, the pressure tapping pipes start to heat up, and there is a concern that this might damage the transmitter, so it is necessary to close the three-valve assembly again completely. I can’t figure out why the pressure guiding tube gets hot. There is steam in the main pipeline, and it’s in use; could it be that when the balance valve and the positive/negative pressure valves of the three-valve assembly are opened at the same time, positive and negative pressures form a circuit through the pressure guiding tube of the balance valve, preventing condensation?
Question 1 was answered correctly, Question 2 was understood correctly
Is it necessary to drain the valve opening sequence of the three-valve assembly? I never empty it; instead, I just close the balance valve and open the valves for the positive and negative pressure chambers. But it’s fine to open the valves during this emptying process as well. However, steam flow meters have one issue: when the condensation tank is not installed levelly, the zero point of the meter does not correspond to a flow rate of zero, and it needs to be recalibrated. It is not recommended to turn on the balance valve before calibration. Regarding question 2, the issue of the pressure tap pipe becoming hot after the balance valve is opened can be attributed to two possibilities. One is that in a pipeline with flow, a slight backflow occurs due to the pressure difference as the fluid passes through the orifice; although this backflow is minimal, liquid does flow within the pressure tap pipe, which is why heat is felt. Secondly, when the condenser tank is not level, opening the balance valve causes fluid to flow from the side with a higher liquid level to the side with a lower level, which results in heating of the pressure tap tube.
1. Step 1 is incorrect: first open the balance valve, then open the positive and negative pressure valves, and finally close the balance valve. Since the range of differential pressure transmitters is generally not large, and the pressure on one side can be quite high, if positive pressure is applied first with the balance valve closed and then negative pressure is applied, it will cause the pressure on the positive side to rise rapidly, which may damage the components. 2. The pipeline gets hot because when all three valves are open, a pressure difference exists, which causes the fluid to flow and hot steam to enter the pressure guiding tube. 3. A short-term increase in temperature of the pressure lead will not affect the transmitter. It can be put into normal operation just like that.
There is no unilateral compression; the balance valve was opened from the start
If the backpressure valve is not in use or is closed, the flow rate should be “0”. If it is not “0”, can this value be set to zero?
……! ! ! 1 is correct; it’s fine to let it vent, as long as the condensate isn’t completely drained! ! It complies with the regulations perfectly! 2 is incorrect; why open the positive and negative pressure valves at the same time when the balance valve is already open? The entire pressure tapping pipeline together with the three-valve assembly form a circuit, right? Any pressure-induced condensate flows into the pipes, and once the water is gone, of course it gets hot! ……As for what you mentioned about opening the positive and negative pressure valves together, this is done without using the balance valve; by opening them simultaneously, both the positive and negative pressure sides are under pressure at the same time, which prevents the instrument from being damaged due to excessive pressure ! This is a common and simple operation! It’s just that opening and closing back and forth is too troublesome; use a balance valve to open them all at once! ! Haha! Is that what you mean? ? This method is essentially equivalent to not taking into account the function of the balance valve! ……But let’s follow the procedures anyway; I recommend the first method!
A basic principle when measuring steam is that the three-valve assembly cannot be in an open state all at once; if it is, the pressure difference between positive and negative pressures will cause condensate to be forced into the pipes...
You should note that the differential pressure created by the throttle device may blow away the condensate in the pressure guide tube.
If you have the balance valve in use, it’s not possible to set the level to zero; if the balance valve is not used, ensure that the liquid level of the condensate in the condensation tank reaches the pipe that leads to the main pipeline (commonly known as the condensate overflow pipe). There should be no flow of fluid in the main pipeline – if steam is required for the process, a bypass can be used – and then the instrument can be zeroed out. Make sure there is enough liquid in both condensation tanks; if it’s difficult to determine, you can open the condensate filling port and check it against an empty reference.