Control's Ask The Experts column is moderated by Béla Lipták, automation and safety consultant, who is also the editor of the Instrument and Automation Engineers’ Handbook (IAEH). If you have automation related questions send them to: [email protected].
Q: We have a 3-in. Samson 3310-07 AT butterfly control valve on steam service with a BR 31a (Pfeiffer) SRP 150-4 (normal close) rotating angle 90° (±4°) and a Capable Positioner 3731-321 00421 100010000 01 served by a 4708 filter regulator. The valve is installed between block valves with bypass.
Maximum flow is 10 ton/h; steam inlet pressure (max) is 17.6 barg (I think this pressure is high for the selected valve, and I will resize and change the selection of the valve); temperature (normal) is 200 °C; and Cv of the valve is 400.
When there’s no flow, the valve works correctly and responds properly up to 15% opening, but when we or the controller call for a greater opening than 15%, it suddenly opens fully (100%). I need your help to understand why this happens.
The positioner has been major issue for me because when instrument air (7 bar) was sent directly, it worked well. When we gave an open command, and after that a close command, the results were satisfactory. This means that the spring is OK.
If the valve opens and closes automatically (spontaneously), then what does it mean? Is the positioner the cause? The process team tells me that spontaneous closing has happened several times without any command on the valve. Right now, the isolation valves are closed, and there is no air signal to the valve, yet the valve is not completely closed.
Samson told me to reset and reinitialize the positioner in its maximum mode. They also suggested controlling the friction torque to open the valve and the tightening (closing?) torque, which could be too high, but I didn’t understand. What do you think?
Rahim Romel / [email protected]
A: First, let me make a few general points. Only at 0° and 90° openings are the pressures equal on both sides of the butterfly disc. Between 0° and 90°, the thrust load on the disc wing that’s turned toward the upstream side is larger than the load on the disk wing on the downstream side. This difference is called the unbalanced, or hydraulic, torque and its magnitude is a function of the pressure drop and disc diameter.
The valve operator must have enough power to overcome the unbalanced torque and, in addition, overcome the friction of the bearings and packing on the valve shaft. The total is known as combined torque. The combined torque required to open the valve is larger than that required to close the valve because the unbalanced torque helps close the valve. This difference is known as torque hysteresis.
In fact, when the symmetrical disc closes, the torque may become negative somewhere around 30–35° from the closed position, and the valve will tend to close itself. The torque characteristics also indicate that at about 75-80° opening, the torques for both opening and closing maximize. Above this rotation angle, the torque falls quickly to zero. Thus, the torque characteristics are highly nonlinear. They pose a considerable burden on the valve automatic operator because it must cope with sharp increases and decreases in torque as well as positive and negative forces.
Typical torque curves for conventional symmetrical, special shape and fluted discs are shown in Figure 1. Manufacturers usually rate the torque requirements of various butterfly valve designs because they’re subject to so many variables. The manufacturer’s recommendations should be followed for selecting the appropriate operator size required to operate a particular valve. It’s best to be conservative when sizing butterfly valve operators because operating and seating torques can often be greater than predicted.
[sidebar id =1]To address your specific question, on steam service, the closed failure position is correct (air to open, spring to close). Therefore, the bearing and packing forces act to resist movement (Fr); the force of air in the actuator (Fa = air pressure x piston area) acts to open the valve; and the spring force (Fs) acts to close it. So, when there’s no steam flow, the steady state force balance is Fa = Fsp. When the valve is ordered to open further, the resistance to movement (Fr) must also be overcome (Fa = Fs + Fr). Finally, if the valve is ordered to open further when steam is flowing, the steam generates an unbalanced hydrodynamic torque force (Fh), which is also trying to close the valve (Fa = Fs ± Fr + Fh).
If the valve works OK up to 15% opening when there is no steam flow (Fh = 0), but fully opens above that, it means that at 15%, the resistance to the actuator torque (Fr) drops. This can be caused by mechanical binding due to pipe strain or disk growth and shaft bowing due to thermal expansion.
If at a certain opening the valve spontaneously opens or closes (“stick and jump” response can also occur due to overtightening packing glands), that means that either the actuator (Fa) or the spring force (Fs) has reached its limit. In other words, you probably have an undersized actuator on an oversized valve or a weak spring (or probably both). In my experience, this is likely to be the case when somebody oversized the valve (you have a Cv of 400 and need only about 100) or when you have mechanical binding.
Béla Lipták / [email protected]
A: If you have a 1 barg pressure drop, the max Cv needed would be 105, and for a pressure drop of 10 barg, the Cv required would be around 45. Based on this, you’re probably oversized.
What may be happening is that the valve is inserted between the flanges and pipe strains act on it, so it can lock up. Since you have a positioner, it will sense that the valve position is not correct, and will increase its air signal until the valve can move—and then it jumps.
Since it’s steam, when the line is operational, it will have normal dilation due to temperature of the pipe, which can put stress on the valve. If the alignment of the pipes is not correct at less than 1 mm from center both vertically and horizontally and the flange holes are not aligned perfectly, then the pipe will put stress on the valve body, causing it to get stuck and not operate properly.
You can also consider reversing the valve body as a temporary solution, as long as the process people approve it. Also, please contact Samson and have them check the valve for your specific needs. They’re best suited to provide answers on possible solutions.
Alex (Alejandro) Varga / [email protected]
A: There can be only five types of failures causing this problem. I ran into similar problems with another manufacturer from Italy on a high-temperature/high-pressure steam application. Eliminate one by one starting with Béla and Alex’s suggestions. The easy and obvious ones are first:
1. Sizing of the actuator to overcome torque to open at high temperature and at both maximum/minimum pressure drop. Check the operation of the positioner for calibration. Also check the direction of flow. When cold, it may operate, but when hot, it may start hunting or drive to extremes.
2. Check pipe line flanges for interference or even gasket mismatch for interference to impede disc movement.
3. Check disc growth due to temperature rise and lack of clearance when hot. I took a cut in the disc size to clear temperature growth. Also, check the thrust plate interference when hot. When the shaft grows it tends to bow. Make sure the shaft is not causing this problem due to growth.
4. Sometimes metallurgy of the bearing material starts binding. If it’s a standard 316 SS bearing, change to Tantalum [soft] or 800 series SS bushing for high temperature. Change the bearing plate for higher gap to the shaft.
5. From your operational description, this looks like a mechanical binding problem
Try all the above in that order.
Ram Ramachandran, Systems Research International Inc. / [email protected]
A: A quick look at the positioner information shows that this is a very complex and sophisticated device, and I suspect that your issues are associated with the positioner programming. If this was my valve, I’d go through the positioner setup procedure very carefully. I suspect it can be set up for many possible behaviors, including as an on/off safety interlock valve. In that case, the action of opening just a little and then opening fully could be an intentional, programmed action.
When you find the problem and the solution, it would be very kind to document it, so others can learn from it.
Cullen Langford, PE / [email protected]