500:1 Liquid Flow Control Turndown Needed 1

[dezirak]

Currently modifying catalyst control system.

Current flow rates vary quite a bit by grade.

.5lbs/hr to 150lbs/hr liquid (0.88 S.G.) is the most striking flow range seen. I am estimating a controllability turndown required of about 300:1 to 500:1.

Current pump set up is a single stage diaphram pump, pulsation dampener, pressure regulator recycle after dampener, Coriolis meter after recycle, then flow control valve. In order to obtain these relatively large turndown ratios and bring the flow control valve OP into a controllable range, operations adjust the pressure regulator setpoint, pump micrometer (and subsequently the pulsation dampener), and tuning of regulator and control valve. This amount of adjustment results in a lot of inconsistency based on the knowledge and experience levels of the people on shift, which in turn may affect our quality due to flow variations. I should mention that the pressure varies downstream of our flow control valves from 60 psig to 140 psig depending on campaign. Maximum regulator pressure upstream of flow control valve is limited to 350 psig.

After speaking to pump vendor as to best course of action they recommended running the pump/recycle loop at a constant high pressure (300-350 psig) and flow while tackling the turndown issue at the control valve level. (An actuated micrometer is not an acceptable option due to limited turndown according to vendor).

Is it possible to get adequate control with a single valve? Is it possible to obtain this level of controllability turndown without resulting to 2 valves ran in split range?

Thanks

 

[LittleInch]

In my experience the max realistic range of a control valve is about 10:1, so I think you will need three, 0.5 to 5, 4 to 40 and 30 to 150 to allow a bit of crossover. The valve will"work" above and below, but at much reduced accuracy, mainly due to the limits of accuracy of the positioner.

I was going to suggest you include a much bigger accumulator and cycle the pump on and off at low flows, but the pump suppliers suggestion is probably as good.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dezirak]

Hmmm... we really don't have the spacing for 3 valves on 9 tanks and that seems like a fairly complicated project with our outdated DCS. I can probably push two per tank through, but 3 per tank would have instrumentation hands ready to strangle me.

I was going through some control forums and one person suggested a course and fine valve operation and trying to avoid split range control. In other words, set the OP% of the course valve and perform fine flow control with the fine valve such that the fine valve is averaging 20-80% OP during control. Once reactors are lined out, we operate at fairly steady rate so I wonder if something like this would be an option. Does anyone have experience with this type of control scheme?

Thanks

 

[LittleInch]

I asusme you're talking about parrallel operation for flow control. Makes it tricky to get the set point right between the two so that you get the flow you want if it starts varying a lot. I think the trick is also to set the response speed of the course valve much slower than the fine valve or just fix it at a fixed percent to avoid hunting between the valves. This means you need to do a lot of things manually.

This is still a hell of a variance in flow though you might get away with two valves in parralel if you can somehow convince the control system to play ball...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Compositepro]

There are also digital control valves that may have eight flow orifices, each rated at twice the flow of the next smaller orifice. The orifices are either on or off and switch rapidly. I've only seen these for benign fluids, like air.

 

[moltenmetal]

You can do better than 10:1 with a single valve but only with the absolutely correct trim selection and a positioner. Fisher's Baumann 51000 claims 1000:1 turndown due to their modified equal percentage trim, but I personally don't find the claim to be credible in realistic terms.

You can also do 100:1 with a pump, between stroke rate and stroke length adjustment- but because of check valve efficiency, you still need the flowmeter.

So you need either two pumps or two 30:1 fine control valves.

A coriolis meter can do 1000:1 turndown- but you shouldn't count on that unless the top flow is absolutely at the top flow end of the meter model selected and the lowest flow is well above the zero stability. Two coriolis meters would be a safer bet due to both pressure drop and practical approach to the zero stability.

I too vote for using a single pump as a pulseless pressure source with pulse dampening and spillback through an adjustable back pressure regulator or better still, a back pressure control valve loop so you can remotely adjust the pressure. Do the whole flow control with the valves. With a 3:1 range on upstream pressure, two control valves with positioners in parallel each with a coriolis meter, you should be able to achieve this.

 

[moltenmetal]

Oh, and don't forget that 4-20 mA signals are capable of only about 12 bits or 4000 counts of resolution- you may get more, but it's doubtful. 12 bits you can rely on. Accordingly, even if you have a perfectly sized high accuracy single coriolis meter, the only way you're going to get reliably 500:1 turndown with any resolution at all is by digital communication- MODBUS, Profibus etc. Two coriolis meters improves that situation quite a bit.

 

[lloydbrown]

The 8 bit digital valve only gets you to one part in 255, although with a 10 bit valve you can get one part in 1023. I would look into an open loop system using a piston or diaphragm metering pump where you control the individual pulses. You would need a stirred inline mix tank to smooth out the concentration. You could use two metering pumps in parallel with a 10:1 stroke volume ratio to minimize the required mix tank volume at the cost of an additional pump and a more complicated computer program.

 

[Boynt]

Keep in mind that there are more choices than pneumatic actuation for the valve. A servo electric control valve actuator can facilitate much more turndown. For example, resolver feedback provides 1000 counts per rev of the motor. Encoder feedback provides 8192 counts per rev. Electrics are very stiff, and if using a roller screw for the linear positioning, can get very accurate at the largest and smallest of flows. The 'positioner' is built in, with 16 bit 4-20 or digital comms.

Barb

http://www.Exlar.com

Best alternative to hydraulic and pneumatic actuation

 

[dezirak]

Thank you all for the input. I think I have a few different options to investigate now.

 

[siretb]

Hi CompositePro

Who manufactures the kind of digital control valves, with multiple orifices you mention?
I googles but cannot find the kind of design you refere to.