Frequently Asked Questions

/Frequently Asked Questions

Selected Answers to questions posted in professional forums


These are usually referred to us directly by noted process control authority and CONTROL Columnist  Béla Lipták as part of his Control Experts Team.

Dear Sir,
I hope you are doing fine. I am Instrumentation Engineer working for a EPC company in Dubai, UAE. I follow your articles closely. I have a query on one of your replies to the query on the subject mail in the below link.
My understanding is that the controller action should be decided based on the control valve fail action position and process requirement. Where  as the control valve fail action position is decided independently considering the safety requirements.

I compiled all my thoughts about this topic together and sending  it for your review. I am still skeptic about the controller action. Can you please review my paper and help me.
I tried to get this concept from books and internet but no where it is complete.
I wish a great week ahead for you.  
Gopikrishna Bathula

China Petroleum Engineering Co.Ltd. ||Instrumentation Engineer||Building No.1,Emmar Business Park, Dubai,UAE||Direct No: +971 44578355||

Attachment: Direct-Acting and Reverse-Acting (with my comments)


Dear Gopikrishna,
I am glad to learn that our column is read in China and congratulations on your good English. Based on your cover letter, it seems that you are correct. Right now, I am I have no time to read your attachment, but will send it to my list of experts and they will advise you,
Best regards, Béla Lipták

Dear Mr. Gopikrishna Bathula.

I have received a copy of your letter from Mr. Bele Liptak and was impressed immediately by the extent of your work. You have put together, mostly correct, a lot of important issues. Well done!

There are few points where, as you have mentioned yourself, that you are still confused and I shall try to lighten these dark topics.

One more personal remark before plunging into the subject: you have put most of the things correct and if I repeat some of your sayings it is more for me then trying to correct your (excellent) work.

In the preparation of my reply to you, I started with the definition of direct/ indirect control action and the related matters as set by Gregory K. McMillan (i). I’ll try to remember to attach these few pages from his work.

First of all, I tend to approach this matter (as well as any other in our field) from the PROCESS point of view. We have to make the process act and behave the way it has to and the instrumentation is there to help US achieve this goal.

We have to look at the whole system where the controller and the control valve, the positioner and whatever other hardware, are only part of.

Let’s follow the process of setting up a control loop.

The process exists, having its own characteristics and requirements.

Based on process and safety consideration we chose the control valve to be Fail Open/ Fail Close (or none!) which also indicates what is the right actuator and positioner for the job.

The very first settings that must be right are the controller action mode and the valve actions. If these settings are not right, nothing else matters. The control loop will not function properly!

Based on the process behavior the controller action should behave opposite, to be able to provide feedback correction.  A direct-acting process is one in which the direction of the change in the process variable is the same as the direction of the change in the manipulated variable and vice versa. The manipulated variable, the output of the controller, is translated to be valve position that generates flow, the set point of a slave loop in a cascade control system etc.

The controller action should be the opposite of the process action unless there is an increase-to-close (fail-open) control valve for which there is no reversal of the valve signal.[i]

The valve action determines whether a 100 percent output signal corresponds to a wide open or a fully closed valve. It also determines the direction of a change in the actual signal to the control valve when there is a change in the controller’s output.

From the instrumentation point of view, the signal can be reversed in several locations along its route from the controller (output) to the control valve and you should decide what best suits your needs and availabilities.

Table 1 (Copied from (i)) summarizes how the controller action depends upon both the process and valve actions and on the signal reversal.Controller Action Dependencies

Table 1: The Controller Action Dependencies

The positioner:

This is a device to enssure the control valve executes exactly what it it is told to do. This is a controller (usualy a PID) whose PV is the Valve Position its SP is the controller (ours) output and its manipulated variable is the air pressure within the controller. The positioner will be direct acting if the Control Valve is “Air to Open” (Fail Close) and reverse acting one if the valve is “Air to Close” (Fail Open).

I hope I was able to set some order into your mind (and thanks to you – into mine as well).

Please fill free to contact me direct for any of these matters or any other within our mutual area of activities.

Sincerely yours


Avihu Hiram.

[i] “Good Tuning: A Pocket Guide”, 2nd edition,  Gregory K. McMillan, ISA, pp5 – Testing Laboratory for Fuel Oils – Viscosity of Ethylene Glycol

Dear Mr. Lipatk
How can I calculate torque of a rotary valve without using manufacturer data? Is there any general rule for torque calculation of rotary valves? I need result of calculation for actuator sizing. Can you introduce a good reference for this issue.
Best regards
A. Rahimi

GOOGLEing your subject reveals some interesting sources. I’ll name few:

–          FISHER “Control Valve Handbook” ( ), Rotary Actuator Sizing (pp 132) has a discussion including “Torque Equations” and two practical tables for easy parameters selection.

–           “Rotary Actuator Applications Guide” ( ), has a special section dealing with “Calculating Torque Requirements” (pp.5) as well as some examples.

–           “Rotary Actuator Sizing, Valdisk and Valdisk 150”, ( ), “Steps 3-6” are dealing with some forms of torque calculations.

If these are not enough – fill free to contact me direct.

(Published in

Dear Sir,
I am working in RCSPL, as a design and detailed engineering company for Oil & Gas sector in India.
Please clarify,
Service: Fuel gas line
Pipeline size: 2 inch
Flow rate: 1000 lb / Hr
In the above condition a orifice meter (concentric square edge) is exist to measure the fuel flow. However client’s are request to install an coriolis flow meter in series with upstream of orifice plate for a back up. Is there any issues to install the same. If not, clarify the installation procedure of coriolis flow meter in the same line.
Thanking you.
R. Muthuganesan

As far as I know and based on my experience with both types of Flow Meters, there are no problems in the required installation. There is nothing special in the installation of the Coriolis instrument. The supplier of the instrument will give you any requirements/ recommendations – if such exist.

Dear respected sir,
I am Muhammad Numan  student of instrumentation engineering   I have (DAE)  and I start more education . So sir I want instruments books and some stuff that help full  for me . plz can send me some instrumentation book by mail…
Actually I need  the books that related to level , flow, temperature,pressure,pneumatics,  hydraulics, electromecahnical, viscosity,gravity and calibration etc. I hope you send me soon as soon  I shall be very thankfull to you…
… Yes I want your recommendation . plz guide me…
Best regards
Muahammad numan
Instrument engineer

Some years ago, when I was in your situation I used to download and store every article, not need to say books which I felt of need.

I am sending you some of these which fell into the categories you have mentioned – but –

Remember that the internet today is a live library and every day more books are put on its shelves.

So – my advice to you – always search this library (the INTERNET) for updated information.

I GOOGLEed “instrumentation and control engineering pdf” and got on the 1st page:

– – Control &Instrumentation Principles

– – Measurement and Control Basics

– – Control Station – Innovative Solutions from the Process Control Professionals

and last but not least

–’%20Handbook%20-%20Process%20Measurement%20and%20Analysis/1083fm.pdf  which is THE “PROCESS MEASURMENT and Analysis” book by … Béla Lipták.

Try it

Dear Sir
We hope you are fine;
I’d like to ask about the engineering practice or Standard to define the instrument air demand for the instrument equipments including:-
–          Control valves
–          Shutdown valves (single acting / double acting)
–          Blowdown valves
Appreciating your support and best regards
Ragab Abdel Fattah.
Senior Instrument Engineer

Your question is annoying in a way.

Why? Because this is a textbook question. A question the answer for is easily available in any article dealing with “Instrument Air Supply Design”.

You could have, as I did, GOOGLE your question and see the piles of available information (just few – to be practical): – Typical PFD for Instrument Air Supply System – excellent short article to start with. – Piping Design of Instrument Air Distribution Systems- including some interesting points – worth the time reading (only 4 pages…).
– – Piping design website – under Instrument air there are some important notes regarding sizing the system (I copy):
Sizing an instrument air system is different than sizing a piping system where there are constant flow rates. Instrument air systems should be designed to ensure the safe and consistent operation of the end devices.

When sizing an instrument air system, there are several different approaches. One approach would be tabulating all the instruments and devices that consume air. However, because so many systems in a plant are dependent on instrument air, it is far better to have too much air available than too little.

Another approach would be to size the system for all instruments consuming air at the same time. As a rule of thumb, a great starting point is assuming each end device requires 2 scfm. Add all the end devices up, account for future expansion and add 10% for leaks and contingency.”

I shall finish this short survey with an excellent paper (on the website of “Chagalesh Consulting Engineers” – ): – “INSTRUMENT / PLANT AIR SYSTEMS” where you can find actually all you need to successfully design, build and operate any air system to suit your needs (“only” 22 pages). This article also gives you the answers to the questions you are asking about the various users air demand and how to calculate it.
Hope I was of help.

Good luck and fill free to come back to me – if you need any additional help (on this subject ot another).

Hello Sir,
How r u. i am Mandar from INDIA. I studied u r book during my Engineering as u r book is BIBLE for All INSTRUMENTATION ENGINEERS ACROSS GLOBE.
I have preserved the book after 15 years of completion of my Engineering as i am also in the field of Control Valves doing BUSINESS OF Control VALVE MANUFACTURING here.
Can u guide me if there is any course of thorough advanced latest PG course anywhere probable In USA for control valve engineering with allied syllabus laos for those who r interested in CONTROL VALVE Engineering.
mandar d. INDIA

From your letter I understand that you are no novice to the Control Valves business.

Trying to help you and put some notes that will be of use to other readers as well – is not an easy task.

I am sure that an engineer with your type of experience, can surf the net and make out tones of free available material to enhance all those areas you feel you need support in.

  1. To find a specific course on a specific subject (Control Valves – in your case) – I would have referred to the internet. While trying it myself, few options were opened:

    a.       CV course

    b.      CV training course

    c.       CV sizing course

    d.      CV engineering

    e.       Fisher CV design course
    and few more – I am sure you can take it from here.

  2. Some material – every Process/ Process Control/ Process Instrumentation professional should have available, which is accessible on the net:

a.       Fisher/ Emerson CV Handbook –

b.      Masoneilan CV Sizing Handbook –

c.       Cashco – Basic Operation and Function of Control Valves –

– to name few.

If you fill you need more specific guidance, please feel free to contact me and I shall try to help you more.

Hope I was of help to you.

Dear Sir,
 Myself is *Ulaganathan. M* Instrumentation Design Engineer
 Sir I am having Doubt,
1. Sir Please Explain me what happens when the Shutdown Valve is kept at the downstream of Control Valve and what will be the impact on it? On what basis or reasons they are installed in upstream of the Control Valve?
2. For the Scenario, In which the Burner System Consists of Fuel Gas System, Pilot Gas System and Air System. In Fuel Gas System Line, there is a Pressure Transmitter with HH / LL Signal Located, our Scope is to install a Shutdown Valve in the Fuel Gas Line, so whether the Pressure Transmitter is Located after the shutdown Valve or Before the Shutdown Valve in the Fuel Gas Line?
What happens when the Pressure Transmitter is Located upstream of a Shutdown Valve and Downstream of a Shutdown Valve? What will be the Impact when doing both type of installations?
Please Explain me Sir. I am awaiting for your reply Sir
Thank You.

  1. You put a shut off valve (SOV) in a point you want no material (fuel in your case) pass in cases you design for. If the control valve is supplying several “customers” and you don’t want to kill all of them, then the SOV’s will be placed accordingly.
  2. Regarding the placement of an instrument (Pressure transmitter – in your case) – you put an instrument in place a reading in needed from. If the pressure is needed to control the fire (as was correctly suggested in the other answer of H S Gambhir) then the right place to put it is as close to the burner as possible. Remember that pressure under “no flow” conditions is either the full system pressure (in case the reading is done upstream the SOV) or null (if it is measured downstream the SOV).

These “rules” are not unique to fuel firing systems and are applicable to all control loops.