6 replies to this topic

[cat]

Hi! I am currently re-evaluating a control valve here in our plant.

The specific control valve is a Neles-Jamesbury G120S type (0.5 in) regulating hydrogen at low flows, i.e. 0-1.6 kg/hr. Based from the technical documents provided, upstream pressure is about 4300-4600 kPaa (from a header) and downstream pressure nearly 2500 kPaa. Average DP (based from actual data) is approximately 1900-2000 kPaa. Temperature is 30C.

Currently, the valve is erratic (scattered data points) - operates at 0-10% opening and registering flows of 1.7 kg/hr, even at 0%. I have looked onto the specifications of the valve, thinking that it might be oversized, but upon recomputation (utilizing Nelprof 5.0 - software provided by Metso, manufacturer of Neles-Jamesbury), it was actually undersized. The installed valve is using a spline trim #12, with Cv of 0.003, whereas the current demand should utilize a valve with Cv of 0.02. This corresponds to spline trim #7 or #8.

I'd like to ask how come that the data I have gathered shows that it is an "oversized valve" but in truth it was actually an undersized valve. What other factors could have contributed to such?

On a side note: We have another set-up identical to this one (we have two "plants"), and the same problem was encountered. The valve was replaced by a Fisher EZ globe control valve and an upstream orifice was installed. Data showed that good control was achieved, operation at 20-60% opening.

Thanks for all the help!

[Zauberberg]

As you said, if the valve operates @ just a few percentages opening/valve travel, it cannot be undersized. As a matter of fact it is greatly oversized and usually what happens with these valves is, that they start experiencing errosion by time (particularly in high dP service such is yours) and there is always some flow through the valve even if it shows as 100% closed - just like you described.

We have the same issue here with another valve but in completely different service: greatly oversized valve that has been operating with 4-5% opening and then started leaking and passing the same flow even when fully closed.

You have found a solution (read your final note) so there is no need to reinvent the wheel. There is no better way to solve a problem than by applying something for which you know that it works 100% in reality.

[fallah]

On a side note: We have another set-up identical to this one (we have two "plants"), and the same problem was encountered. The valve was replaced by a Fisher EZ globe control valve and an upstream orifice was installed. Data showed that good control was achieved, operation at 20-60% opening.

Would you please explain about the size of the line on which the control valve to be installed,the size/type of the Neles-Jamesbury G120S control valve and also the size of the Fisher EZ globe control valve.
How do you obtain the size of upstream orifice/size of new control valve?

Edited by fallah, 06 April 2010 - 01:06 AM.

[cat]

Neles-Jamesbury G120S is a 1/2" globe (Cv=0.003, based from vendor) and the Fisher valve is also a 1/2" globe valve (Cv=0.036, based from vendor).

The size of the "new" control valve as well as the upstream orifice were computed using the vendor software. Control valve sizing inputs were T, P, dP, and Q (as stated from previous post). The orifice was sized in such a way that it will give a maximum flow output of 1.6 kg/hr (which is the max output of the control valve).

Would you please explain about the size of the line on which the control valve to be installed,the size/type of the Neles-Jamesbury G120S control valve and also the size of the Fisher EZ globe control valve.
How do you obtain the size of upstream orifice/size of new control valve?

[fallah]

Neles-Jamesbury G120S is a 1/2" globe (Cv=0.003, based from vendor) and the Fisher valve is also a 1/2" globe valve (Cv=0.036, based from vendor).

The size of the "new" control valve as well as the upstream orifice were computed using the vendor software. Control valve sizing inputs were T, P, dP, and Q (as stated from previous post). The orifice was sized in such a way that it will give a maximum flow output of 1.6 kg/hr (which is the max output of the control valve).

Delta P across control valve in new case can not be the same as old one,because adding orifice results in less delta P accross control valve.Please clarify.

[cat]

yes.

dP across the valve varies as the % opening is varied. the changes can be characterized by the pressure ratio factor, DPm. This is the ratio betwen dP across the valve max flow rate to dP across the valve when fully closed. DPm was computed by the software. Applying the definition - since we know the dP across the closed valve, we then calculated the theoretical dP of valve at max flow rate, which is lower than the current values we are obtaining. So the orifice was designed based on the max flow rate and "calculated" desired inlet pressure (which will be the outlet P of the orifice).

Delta P across control valve in new case can not be the same as old one,because adding orifice results in less delta P accross control valve.Please clarify.

[fallah]

So the orifice was designed based on the max flow rate and "calculated" desired inlet pressure (which will be the outlet P of the orifice).

For me a little bit confusing,therefore i think the following statement may better clarify the issue:

"The orifice is designed based on the max flow rate and delta P can be obtained by subtracting calculated the theoretical dP of valve at max flow rate from whole desired delta P across control valve system (valve+orifice)."

Actually orifice absorbs additional delta P results in control valve can operate between proper range of opening percents.

Please comment (if any ) on my above understanding.

Edited by fallah, 07 April 2010 - 01:31 AM.