This column is moderated by Béla Lipták, automation and safety consultant and editor of the Instrument and Automation Engineers' Handbook (IAEH). If you have an automation-related question for this column, write to [email protected].
Q: Selecting valve flow characteristics: It’s always been encouraging to get guidance from you. I have been studying different authors’ guidelines concerning control valve characteristics. Referring to your book (Process Control and Optimization, Vol. II, Fourth Edition), I need your guidance.
For the selection of the control valve inherent characteristics for flow control, Table 6.1g gives different recommendations for applications "wide range of flow set point" and for "small range of flow but large ΔP."
My query concerns the definitions of wide and small ranges in terms of flow turndown (e.g 5:1, 2:1, etc.).
-- Kaushal Shah, Instrumentation & Control Systems Dept., [email protected]
A: A control loop will be stable if its gain does not change with variations in the load. The loop gain is the product of four gains (process, controller, sensor, valve) and, ideally, it should stay constant at about 0.5. If the process gain varies with load and the sensor and controller gains are constant, the ideal valve characteristic is one that will compensate for the variation in process gain, so that if the process gain rises with load, the valve gain drops (equal percentage); if the process gain drops with load, the valve gain rises (quick opening) and if the process gain remains constant, the valve gain should also be constant (linear).
As to the valve characteristics for flow control, less sophisticated users tend to specify linear valves for all flow applications. Sophisticated users for critical flow control applications tend to run dynamic simulation tests to determine the best valve gain fit to obtain the ideal gain characteristic for the loop. Here, my answer assumes an average, less critical application.
The inherent valve characteristic is the relationship between stem position and flow at constant pressure drop. When the valve is on the discharge of a constant-speed pump and it is closed, the valve pressure drop is maximum (F=0, ΔPmax). As the valve opens, flow rises, which increases the back pressure and the pressure drop available for the control valve (ΔPvalve) drops. Therefore, at maximum flow, the valve pressure drop will be the minimum (Fmax, ΔPmin).
In my experience, if in a particular application the ratio ΔPmax/ΔPmin is less than 2, a linear valve can be used if the flow is being detected by linear flow sensor detectors, and a quick opening (QO) control valve can be used if the flow sensor is square-root (Figure 1).
If in a particular application the system curve is steep and ΔPmax/ΔPmin is greater than 2, the typical selection is equal-percent (=%) characteristics when the flow detector is linear, and linear characteristics can be selected if the flow detector is square-root (Figure 2).
I should note that the above guidelines apply only to constant-speed pump applications. Also, in some applications, the piping configuration is more complicated, such as when the valve is not in series with the flow detector, but in a bypass around the flow sensor, so that when the valve opens, the flow drops.
-- Béla Lipták, [email protected]
Q: Flow measurement of hydrocarbon with sand? I want to measure hydrocarbon condensate flow where the condensate contains about 1% sand on a mass basis. How would this affect the measurement if a Coriolis or a vortex flowmeter is used? The sand comes from the wells, and is collected and removed downstream of the flowmeter in the liquefied-petroleum separator.
-- Azri Syahmi, [email protected]
A: Either Coriolis or vortex can wear at a rate proportional to the percentage of sand in the flow, and both meters could have a short lives. Sharp-edge orifices are similarly vulnerable. The choice is affected by flow rate and pipe size.
I would worry about any Coriolis flowmeter with a relatively thin metal wall. Magnetic, ultrasonic and flow nozzles have been recommended. With a sufficiently high flow rate, an elbow meter will also work. Any DP meter might need a chemical seal or liquid purge to avoid plugging.
-- Cullen Langford, P.E., [email protected]
A: I would love to tell you that you can use a Coriolis flowmeter, since it would be unaffected by sand entrained in the flowing hydrocarbon, but the resulting mass flow would be biased by the sand. It is almost impossible to instrument/measure/control a poorly designed process. Remove the sand first, then measure flow after the sand removal. Fix the process first, then choose where you install instrumentation.
-- Richard H. Caro, CEO, CMC Associates, Certified Automation Professional (ISA), [email protected]
A: Most flow measurement equipment, such as vortex and Coriolis, would work quite well. Having said that, here are the caveats:
1. You need to periodically inspect the flowmeter internals to make sure abrasion has not damaged the vortex-shedding block or the tubes inside the Coriolis.
2. You need to verify periodically the type of sand particles to make sure that silicates or other hard rocks are not increasing, because this could affect the abrasion risk.
3. You need to periodically verify that the quantity of sand is not affecting the measurement. You have to remember that the solids displace the oil/water mixture and can affect the final measurement.
4. Since you have sand, I would use vortex meters, even if the measurement may not be as precise, since they are cheaper to replace.
-- Alex (Alejandro) Varga, [email protected]
A: This is a common measurement problem in coker feeds (sand and clay up to 5% to 10%) from column bottoms in oil refineries. If you can use a solids collection pot before entering the straight-pipe mass flowmeter, you can read the flow reliably until solids appear in the flow tube.
You will experience noise problems if using target or vortex meters. The best choice would be to select a mass flowmeter installed in a vertical pipe with a solids removal arrangement. Periodic draining is the simplest way to solve your application problem. Good maintenance will enable reliable measurement.
-- Ram Ramachandran, [email protected]
A: A Coriolis meter is not the first meter that would come to mind for a fluid with solids. It may technically be able to measure the flow with 1% solids, however, I would be very concerned about erosion, particularly for the U-tube-style meters. The only possibility would be the straight-type meters, but I would recommend that you check this with the vendors. In recent years, they have improved the coil/tube drive technology to better cope with energy losses due to two-phase flow (gas/liquid). But the tubes in Coriolis meters are thin, and cannot cope with erosion or corrosion.
Vortex meters may handle a small amount of solids, but this is not ideal (vortex are usually for clean service). Check with vendors on how much erosion of the bluff body is acceptable.
I would consider going back to old-school venturi or wedge flowmeters, but not orifice. It would be best to install in a flow-up configuration (avoid trapping of solids). If that is not possible, then a wedge flowmeter would be OK in horizontal flow with the tip of the wedge facing down (to not trap the solids). Since the wedge is for low Reynolds numbers, you would have to check.
There are other things to consider for avoiding plugging of impulse taps.
-- Simon Lucchini, CFSE, MIEAust CPEng (Australia), Chief Controls Specialist, Fluor Fellow in Safety Systems, [email protected]
A: Any in-line flow meter is likely to be affected by sand in the hydrocarbons. Why not try a clamp-on ultrasonic flowmeter? Vendors often have very good application engineers who will assist customers with challenging problems.
-- Raj Sreenevasan, [email protected]