18 replies to this topic

[Jiten_process]

hi all

I need a firm figure from you all. I am carrying out air consumption calculation for our small project. I have three control valves and three pneumatic on/off valves. My doubt is i could not get the air consumption figure for each control valves; more over is it continuous requirement or just at a time of actuation?

Two nos. of controlvalve for steam application with sizes 50NB and one no. controlvalve for water application with size 65NB.

Plz reply....

[ankur2061]

Jiten,

I am attaching a small excel sheet which gives an estimation for air consumption of various types of pneumatically operated instruments. Please note that although practically in a steady state operation there is no actual consumption the table attached tries to quantify air consumption on a continuous volume flow basis.

The tabular numbers have been picked up from some company guidelines and please do not ask me to reveal the source since revealing the source is a breach of confidentiality. As an abundant precaution, I have mentioned that the air consumption for any particular type of instrument should be verified with the instrument manufacturer/supplier.

Regards,
Ankur.

Quick note from the admin: This attachment is so good, we've moved it to our File Library

Edited by Chris Haslego, 19 November 2012 - 02:14 PM.
Moved to File Library

[minyee04]

In my project, the control valve's normal air consumption has to be considered as 10 SCFM due to leakage. This is a Flow Control Valve at about 2" size. It is working on steady-state and is provided by our client. Did your excel spreadsheet consider the leakage of the control valves? Actually, I am quite new in this project. I am still cracking my head whether to calculate the average air consumption or total consumption at one time so that my instrument buffer vessel and air compressors are able to cope with the large total air consumptions? What is the basis to be used?

[DRS]

Firm figures depend upon your process requirement, IMHO. What others can suggest is a ball park figure. 5scfm is fairly OK for control valves and I would treat that as continuous requirement. For On/Off valves, it is intermittent. I use 1.5scf for one valve operation (one open+one close) and try to arrive at flowrate by no. of valve operations in one hour.

The link below has excellent tips on receiver sizing.
http://www.pyebarker...ps/techtip3.htm

[ankur2061]

Hello,

As I have very categorically mentioned, the air consumption for any instrument has to be ultimately verified by the instrument supplier/manufacturer. Most reputed control valve vendors such as Fischer, Valtek and others will be definitely able to provide the maximum air consumption if asked. The spreadsheet that I have provided encompasses some broad guidelines based on some company standards. No details about the exact mode of air consumption is provided such as leakage or any other mode. However, you are free to use whatever data has been provided to you if it concurs with the manufacturers recommndations for air consumption.

Hope this helps.

Regards,
Ankur

[Mehrdad]

Hi Jiten

the air consumption for control valves is different and a function of:
1-supplier
2-valve system
3-line pressure
4-work conditions

in 1-3 items the rate of consumption is constant but in 4th item, the variation in work conditions is a factor of air consumptin.
on/off valves need continuously air pressure and no air consumption(except a little consumption due to leakege) and in air flow disconnection, the position of valve will set to normal position that its depend on that the valve is normal close or normal open.
therefor you need to talk to your supplier.
cheers

[RZD_Tripatra]

Usually vendor will give data steady state consumption and maximum output capacity.
Please do not use maximum because we will get oversize consumption.

Just use the steady state consumption + 20 % contingency. It will enough. For easy calculation you may state that each control valve consume 1 SCFM (vendor usually give less than this value in steady state consumtion, but it's still OK).

Best regards,
Rizaldi

[Jiten_process]

Thanks a lot all

Actually now this has become an issue for me. I could not get authentic data which is suppose to be given by vendors. Different vendors giving different data considering higher size marzin. I dnt want to oversize my compressor at all.

we have been taking 7 Nm3/hr so far but it is apparantly seem so high. Let me give u some inputs which is given to me by vendor. May be others can throw more light now.

I got following data from vendor

For control valve,
1.4 bar pressure
0.15m3/hr for 6mm tubing, 0.225m3/hr 8mm tubing, 0.45m3/hr with 12mm tubing.

for 6 bar pressure
0.5m3/hr -6mm tubing, 0.75m3/hr-8mm tubing, 1.5m3/hr-12mm tubing

Above is what exactly vendor has given me in mail. From this now i have to assume lot many things.
firstly i assumed that he is talking flow at specified pressure not but Nm3/hr
second, 1.4bar pressure he must be referring after Air regulator

Does anybody have a familier feeling with these figures, is it seem ok?
to be continued....

[ankur2061]

Jiten,

Would you be kind enough to tell us, which vendor has provide the data.

I am a little surprised that the vendor has given the consumption based on air tubing size. Is the consumption based on leakage by rupture of the tubing?

As far as consumption in m3/h is concerned that seems justifiable since air is to be supplied at some pressure to the pneumatically operated diaphragm of the valve. The 1.4 barg and 6 barg seem to be the minimum and maximum pressure limits for operation of the control valve diaphragm.

As I undertsand, the volume of the pneumatic cylinder/diaphragm of a on-off/control valve play a role in determining the air consumption. For a full stroke of the control valve (0 to 100%) opening for an air-to-open control valve the volume of air required to fill the pneumatic diaphragm can be found out. With some reasonable assumption of the maximum number of strokes that can occur in a unit time period (say one hour), the volumetric flow rate can thus be determined.

Have you checked out with some reputed vendors like Fischer (Emerson), Valtek, Masoneilan for air consumption for your type of control valve. I would tend to believe some of these internationally known vendors than some local vendors who have been duplicating design of international vendors without having the thorough knowledge and insight of control valve engineering.

As a summary, I fail to understand your insistence on exact numbers for air consumption for the following reasons:

1. If as an exact calculation you have determined that your total air consumption is X Nm3/h, it is highly unlikely that an air compressor vendor would be able to supply you a compressor with your exact requirement of X Nm3/h. Air compressor vendors nowadays, have standard packages with a combination of flow rate and pressure and designated as model numbers. No air compressor vendor offers of-the-shelf compressor as per the customers exact calculations for air consumption. Essentia;lly it means that if the air compressor vendor has one model which gives a flow rate of 1.15X Nm3/h and the next lower model which gives 0.9X Nm3/h, you are not going to get into an argument asking him for a model with the exact flow rate of X Nm3/h. The most sensible thing I would do is just go ahead and buy the model with the flow rate of 1.15X Nm3/h.

2. If you look at the vendor catalogs of air compressor vendors, many a times two different model numbers (numbers close to each other) having small difference in flow rates and the same discharge pressure have the same motor rating which ensures that your OPEX remains the same between these model numbers.

Personally, I feel the topic has no merit for further discussion based on my above points for air compressor selection.

Regards,
Ankur.

[Jiten_process]

Thanks for your practical suggestion ankur,

let me tell you why i am insisting the air flow consumption for control valve, actually we devide the total air consumption in two different classes continuous and non continuous and we size the compressor based on continuous requirement, for non continuous we size air receiver so as to cater peak requirement which is for small time. Now i consider air bleeding across control valve is continuous requirement and when we talk about 50-60 nos. control valve in whole plant, it becomes substatial figure. We have been taking it as 7Nm3/hr but i found it should be nearly 2-3 Nm3/hr, now you can emagine if my latter finding is true and when there are 50 nos. control valve, imagine how oversize my compressor would be.

Now as far as compressor selection is concern, i agree that i cant get the compressor exactly of my size, rather i would select next higher size, but in above case it becomes substantially oversize, which i would not accept.

neways, you are right, it's not benificial enough to stretch this topic anymore. let us close the discussion.

And once again thanks a lot to all for your valuable inputs.

[Qalander (Chem)]

Dear Jiten Process,

Pardon me for entering into the discussion, but I feel like adding my little comments as follows gathered from my almost 28 years stay at my previous employer 3 refineries complex mainly Pnematic intruments/Control valves and many a times witnessed problems during rotating shifts.

I do not fully endorse that compressor sizing will be dependent on continuous flow as this figure really should vary with

the Mechanical Integrity/efficient keep-up level of Control valves etc.

Initial needs estimation of intrument air has to be addressed through the adequate reciever sizing even if over sized [u]

as this is usually less costlier as compared to the compressors.

Thus spare/stand-bye compressor(s) are sized in such a way

to cut-in for header pressure drop compensation(s)

as and when operational need arises.

Hope the above helps.

Best regards for all colleauges as these healthy arguments open hidden facts for everyone's awareness indeed.
Qalander

[pseudocomponent]

Hi Ankur,

In your exel file above what's your standard reference conditions of temperature and pressure?? Is the reference temperature 0 degr.F/15degr.F/20degr.F etc.?? Of course i know the pressure is usually 1 atm..

[ankur2061]

pseudo,

SCFM / Sm³/h refers to a condition of 60 deg F (15.56 deg C) and the pressure is 1 atm abs (1.01325 bara)

Hope this clarifies.

Regards,
Ankur.

[skhattak]

Dear Ankur,

It will be so kind of you, if you can please tell me the exact procedure of calculating the air consumption rate of control valve (both for on/off and throttling).

Best Regards,
Shakeel Khattak

[ankur2061]

Shakeel,

The procedure as such is not very simple since it involves the volume of the cylinder / diaphragm for the on-off / control valves, the time-cycle (frequency) of operation of the valve as well as the leakage rate from the cyclinder / diaphragm. Here only one factor is easily determinable which is the cylinder / diaphragm volume with the other two very difficult to estimate.

The excel sheet provided in this post gives fairly conservative figures for air consumption and one could always provide a safety factor on the calculated air consumption so that the instrument air package is not undersized and may even be able to take care of future modifications / debottlenecking of the plant. Shell, for example recommends a contigency factor of 1.3 on the calculated air consumption.

I don't see the need to do such detailed calculations based on the procedure I have mentioned above. I would consider it a luxury to spend time on such detailed calculations.

Regards,
Ankur.

[jugal]

Dear sir,
Can u provide me information for Pneumatic valves working on basis of solenoid valve.

[ankur2061]

Jugal,

Any pneumatic valve has either a diaphragm or cylinder system for actuator movement. Solenoids only energize and de-energize to facilitate either supply of air or venting of air. I don't see what difference a solenoid is going to make on the air consumption of a pneumatically operated control or on-off valve.

In case you are so particular about obtaining exact values of air consumption as a flow rate then the best way is to get the information from the manufacturer / vendor of the pneumatic valve. Otherwise, a conservative estimate (higher consumption figure) is fairly acceptable.

Regards,
Ankur.

[Qalander (Chem)]

Dear Jugal Hello/ Good day,

• I agree to and fully endorse Ankur's above comments.

[Pbhadane]

Ankur,
Thanks lot for the detail information provided in this forum.