18 replies to this topic

[Shahzad Ahmad]

Dears,

 

Hello everyone. I need your suggestion/endorsment for the following air calculations.

 

We are Installing a Reverse Osmosis Plant in our Company. The Vendor have calculated the Instrument Air Requriment as 300 NMC/h (Normal metric cube per hour). The Control valves will operate with this Air at 7 bar Pressure.

 

Using the formula "P1V1/T1=P2V2/T2, I suggest that 50 m3/h is the required air flow at 7 bar, and 35 C. But the others are of the view that the operating pressure has nothing to do with Air quantity and 300 figure remains constant at different operating pressures.

 

I need your suggestions. Kindly suggest me any reference with industrial practice endorsement.

 

Regards,

 

S.Khan

[breizh]

S.Khan ,

You need to check the consumption required per valve , should be indicated on the data sheet of the valve(s) and then perform the calculation for the entire set of valves and compare with the data given by the vendor (300 Nm3/h.


Good luck.

Breizh

[Shahzad Ahmad]

S.Khan ,

You need to check the consumption required per valve , should be indicated on the data sheet of the valve(s) and then perform the calculation for the entire set of valves and compare with the data given by the vendor (300 Nm3/h.


Good luck.

Breizh

Dear Breizh,

 

I am not about to verify the air consumption calculated by Vendor, 

But I am trying to interpret the 300 NMC/hour at operating conditions, calculated by vendor. At operating pressure of 7 bar the air flow rate will minimize as per the formula P1V1/T1=P2V2/T2. Am I right, because my supervisor says that the flow rate will be 300 m3/hr 7 bar. 

 

 

Regards, 

Edited by SAK..., 27 May 2015 - 02:27 AM.

[Zauberberg]

Air consumption is normally given in Normal or Standard units (these conditions need to be defined because they do vary from one project to another) for easier calculations.

 

Remember that in the Instrument Air network all consumers will be receiving Instrument Air at a slightly different pressure, due to pressure drop in the IA system, from beginning to end. Whilst the consumers located closer to the Instrument Air vessel will be seeing pressure close to that of the vessel, the consumers located at the far end will be seeing much less pressure - it could be up to 1 bar difference, or even more. Using standardized flow unit (Normal or Standard volume flow), calculations become much easier and straightforward. Standardized volume flow can be easily converted into mass flow unit.

 

Therefore, you don't need to perform any re-calculation at actual pressure and temperature. Standard m³/h is always the same figure, regardless of the operating conditions, because it always refers to a standard value of pressure and temperature. If you are looking at calculating Actual volume flow, then you need to apply pressure and temperature correction.

[Shahzad Ahmad]

Dear Zauberberg,

 

Thank you for your response, but let me further clarify the background of my problem because what I want to know is not what are u suggesting...

 

We are Installing Reverse Osmosis Plant at our Plant. The Vendor is responsible for membranes skids and related auxilliries. The Vendor's have calculated Requirment for the said plant as 300 Nm3/hr. 

We are responsible for arranging Air compressor for the above air requirement. Now we are contacting different Vendors for air compressor provision. We have asked for 300 Nm3/hr air flow, and also the operating pressure of 7 bar.

 

My point of concern is, 

1. At operating pressure of 7 bar, the flow less than 300 m3/hr, as per the formula described above. While My supervisor says that at 7 bar pressure we need 300 m3/hr.

 

Kindly suggest me any practical literature or reference with which I can understand the whole problem..

 

 

Regards.

[samayaraj]

SAK,

 

The term Nm³/hr indicates flow at normal operating pressure & temperature. Hence for Pressure 1.013 Bar a & temp 273K, the flow rate is 300 m³/hr also called as 300 Nm³/hr. As you said, for 7 bar pressure the flow will be less at the same temperature.

 

If you are seeking for a flow rate of 300 m3/hr at 7 bar, you need to mention at what temperature you need the required flow rate, since as temperature varies, volumetric flow rate will vary...

 

Look at the excel sheet for sizing instrument air line sizing.

 

It may be useful in giving solution to your problem.

Attached Files

•  3. Instrument air line sizing- Sam.xlsx   31.33KB   167 downloads

Edited by samayaraj, 27 May 2015 - 04:33 AM.

[Zauberberg]

Normal flow is normal flow, and it does not change with pressure. If you have the requirement for 300 Nm³/h of air, this figure (in terms of equivalent mass flow, kg/h) will be always the same, irrespective from the operating pressure.

 

The actual flow at 7 barg will of course be less than at standard/normal conditions. It cannot be equal to 300 Nm³/h. Your supervisor is wrong.

[ankitg009]

In complete agreement with Zauberberg.

 

WIth increase in pressure , the volume will decrease(as per gas law).You were right.

 

@Samayaraj

NTP conditions are 1 atm and 20ºC(293 K).

[Zauberberg]

NTP conditions are 1 atm and 20ºC(293 K).

 

The problem with NTP and STP is that they are not uniform across the globe. What a good engineer will always do, is to specify the pressure and temperature to which these conditions refer to.

 

I have seen standard conditions at 1 atm and 15 degC, 1 atm and 20 degC, 14 psia and 65 degF, etc. Different sites have different conditions. The same applies for normal conditions. There are sets of values which are used more often than the others, but that does not make them universal.

 

Make sure you aways have the Standard/Normal conditions clearly specified when you use these units in Heat & Material Balances.

[ankitg009]

 

NTP conditions are 1 atm and 20ºC(293 K).

 

The problem with NTP and STP is that they are not uniform across the globe. What a good engineer will always do, is to specify the pressure and temperature to which these conditions refer to.

 

I have seen standard conditions at 1 atm and 15 degC, 1 atm and 20 degC, 14 psia and 65 degF, etc. Different sites have different conditions. The same applies for normal conditions. There are sets of values which are used more often than the others, but that does not make them universal.

 

Make sure you aways have the Standard/Normal conditions clearly specified when you use these units in Heat & Material Balances.

 

Valuable info. Thanks

[samayaraj]

Thanks ankit for your correction...

Ya its 293k for NTP.

As Zaberberg said, I have faced the same many times. So I do mention the pressure and temperature irrespective of STP or NTP.

Edited by samayaraj, 27 May 2015 - 05:49 AM.

[Shahzad Ahmad]

Thank you all for your kind suggestions...

[breizh]

SAK ,
Sorry If I did not pick up your query , probably good for you and other to read the document attached .



Breizh

[Chemical.Engineer]

Hello

I have 2 questions about instrument air system:

 

1- The required air for each control valve depends on control valve size?in our system, the required air for each control valve is considered 1.3 sm3/h but i see Larger quantities in different references. It could be because size of control valves in our plants is smaller than conventional oil and gas plants?

 

2- in sizing of air network, how much should be the minimum required pressure for each user and maximum velocity?minimum 4barg and maximum 20 m/s are ok?

[shantanuk100]

Dear SAK,

 

Please read through the following for an understanding of the conditions.

 

A.  NTP And STP are conventional systemized conditions uniform throughout the world under any surrounding condition of P,T etc.
B.  They are independent of the conditions in the site or elsewhere.

C.  They are just a way of reporting the flow required in a pre-specified already known and set conditions so that they can be uniform throughout the globe in case you have to report the values to someone else, since you would be unaware of their flow conditions.

So they can then determine the actual flow rates from either the known Std. or Normal flowrate.

 

1. Conditions

 

In chemical engineering, as per API 520-1 , 2008 (5.6.4.1) , the normal conditions are as defined below.

 

- NTP is typically 0 oC and 1 atm (101.325 kPa) though it varies a lot as per the convention adopted for your case, so it is better to confirm with the respective people involved.

- STP is usually 15.6 oC and 1 atm (101.325 kPa) though it varies a lot as per the convention adopted for your case, so it is better to confirm with the respective people involved

 

In a lot of cases the conventions are taken differently but this is as per API so if no other convention is mentioned in your case then you can consider the above API conditions for calculations.

 

2. Real Gas Law

 

You cannot assume ideal gas for your calculations since it is a real case. 

So you cannot apply P1*V1/T1 = P2*V2/T2.

You must consider the compressibility of the Gas at both the respective conditions 1 and 2.

So the Real Gas Law must be applied here as follows.

P1*V1/(T1*Z1) = P2*V2/(T2*Z2)

 

3. Conversion

 

Since the Flow of Air has been obtained to be 300 NM3/hr, it means that for a condition of 0 oC and 1 atm(1.01325 bar) pressure you get a flow of air to be 300 m3/hr.

So now to get the real flow at your operating Pressure and temperature you convert this using the real gas law.

P1*V1/(T1*Z1) = P2*V2/(T2*Z2)

 

4. Calculation Using Compressibility factor of Air

For your case you can consider this source for Air (https://en.wikipedia...sibility_of_air)

From the source you obtain 

Z1 = 0.9996 (at 273K and 1.01325 Bar which are the Normal Conditions of Flow)

Z2 = 0.9965 (at 308 K (35 oC) and 7 Bar, which is your operating condition of Flow)

 

So you get,

 

(1.01325 Bar * 300 Nm3/hr) / (0.9996 * 273 K) = (7 Bar * Q m3/hr) / (0.9965 * 308)

 

This gives Q = 48.84 m3/hr at 35 oC and 7 Bar pressure.

 

5.  Conclusion

 

Your flow is 300 Nm3/hr at 1 bar, so it simply cannot be 300 m3/hr at 7 bar as well for a very small temp. change of 35 oC. Your supervisor seems to have misconsidered some variable so show him the procedure for calculation. 

If your Control Valve was operating at Normal Conditions of 1.013 bar and 0 oC, then your supervisor is correct in saying 300 m3/hr will be the air flow rate.

But since the conditions change, the above way is the correct way to calculate it, and you obtain 48.84 m3/hr as the flow.

 

Regards,

Shantanu

Edited by shantanuk100, 23 November 2015 - 04:50 AM.

[samayaraj]

Hello

I have 2 questions about instrument air system:

 

1- The required air for each control valve depends on control valve size?in our system, the required air for each control valve is considered 1.3 sm3/h but i see Larger quantities in different references. It could be because size of control valves in our plants is smaller than conventional oil and gas plants?

 

2- in sizing of air network, how much should be the minimum required pressure for each user and maximum velocity?minimum 4barg and maximum 20 m/s are ok?

 

1. For more work done, you need to supply more air.

 

2. Generally, vendor recommends min of 4 to 5 bar g pressure for satisfactory operation of valve. 20 m/sec velocity is more and there will be more carryover inside pipe due to high velocity. You can go upto 10 m/sec. If you go below 10 m/sec, it will be more safe. Lower the velocity, less is the carryover of moisture etc etc.

[56200358]

Hello

For specifying of air compressor capacity in a  plant, is there a approximate relation between instrument air consumption and service air consumption?How much should we consider service air consumption for a plant? (we have amount of instrument air consumption)

[shvet]

Hi 56200358

 

As I know there is no simple rule of thumb. Service air consumption depends on a lot of options and varies from plant to plant. You should provide us with more information about what kind of plant you have and which types of units there are. And even then it might be useless. Best way would be to obtain annular servise air consumption of the same type plant (more or less capacity).

[breizh]

hi 56200358 ,

 

No rule of thumb , you need to know the quantity used per equipment , the service ( continuous or discontinuous) , most probably you will have air for instrument and plant air for equipment ( pump, actuators,etc )  . At the end you will need to integrate a safety factor because  you cannot be short of air  !

 

Note : Most probabaly you will have a spare equipment ....

 

The first document you need is a PID and then start to count the users .

 

Good luck .

 

Breizh