7 replies to this topic

[mm217]

Dear All;

1.As we all know Screw compressors are in positive displacement air compressors and shut off pressure doesn't mean for these compressors. at the beginning of design how can i estimate the design pressure of these compressors?

2.In our project, we have air compressor and it seems fire case is rare, so block outlet is applicable. can we say because block outlet based on 10% accumulation, design pressure would be 10% over than max pressure?!

3.How about design temperature?off course compressors have intercoolers and after coolers. should we concern cooler media failure and consider maximum temperature outlet as base of design temperature (max temp+20 or 25 C) or we should imagine that max allowable temperature of compressor outlet and then calculate design temperature? can we use HYSYS?

[ankur2061]

mm217,

You are complicating the issue for the design conditions of a compressed air system.

In a majority of cases compressed air systems for plant air and instrument air operate at a pressure of 7 barg (100 psig). For systems operating at this pressure level 150 # rating piping is generally employed.

Since compressed air piping is above ground and uninsulated it can be exposed to solar radiation. The black bulb temperature corresponding to the maximum solar radiation (80-85 deg C) can thus be considered as the design temperature. Typically in the middle-east a black bulb temperature of 82°C is considered as design temperature for uninsulated, above ground piping which is operating at maximum ambient temperatures of 45-50°C.

From the ASME B16.5 tables for pressure and temperature rating for Material Class 1.1 (Carbon Steel) and 150 # rating the design pressure calculated against a design temperature of 82°C works out to be 18 barg. Thus the design pressure of the equipment such as air receiver and 150 # rating piping connected downstream of the compressor system can be specified as 18 barg and the design temperature as 82°C.

For most air compressors providing compressed air supply at 7 barg (100 psig), the operating pressure at the compressor discharge connection is generally in the range of 10-11 barg (145-160 psig). The higher pressure at the discharge comnection is required to account for the pressure drop in the connected air filtration and air drying unit so as to obtain a pressure of 7 barg (100 psig) in the connected receiver / piping system.

Compressor manufacurers provide enough safeguarding in terms of alarms and trips for the compressor if the inter-stage cooler / after-cooler or lube oil system fail to function for the screw compressor.

Compressor manufacturers in fact don't require to be provided the design pressure or temperature for the compressor package itself. They are quite capable of deciding the right design pressure and temperature if you can provide the operating and design pressure / temperature for the downstream connecting equipment and piping. This also depends upon whether the compressor manufacturer is a reputed name such as "Atlas Copco", "Kaeser" and "CompAir".

Hope this helps.

Regards,
Ankur.

[S.AHMAD]

1. Design pressure (pressure vessel) is normally determined from the maximum possible operating pressure. Then add 10% or 25 psi whichever is greater.
2. The 10% accumulation is allowable over pressure for sizing the PSV. The PSV set pressure is the design pressure as determined in item 1 above
3. Similarly with temperature, you need to anticipate the maximum possible temperature. As mentioned by Ankur, the solar heating would be the credible scenario. Normally I add 25F to eliminate uncertainty in operating conditions.
4. Also as mentioned by Ankur, for compressed air (instrument/utility), 150# flange coupled with CS Schedule 40 is more than sufficient in most cases.

[mm217]

Dear Ankur & S.Ahmad;

Thanks a lot for your replys. I understand most of notes you mentioned above. But for more clarification please consider:
1. If we put after-cooler out of compressor package, the compressor discharge temperature would be almost high (even more than 100 C) so, in case of failure of coolant media the temperature would rise higher than black body temperature (?!). How should we define design temperature in that case?

2. Based on AHMAD comment, we can estimate design pressure of positive displacement compressor by adding 10% or 25 psi to maximum operating pressure whichever is greater.Please confirm it again if we can use this method for any positive displacement compressor.

[ankur2061]

mm217,

Why would you want to remove the after-cooler from the vendor's scope? I have been involved in evaluation of many air compressor packages using screw compressors and never have advised or recommended for the after-cooler to be taken out of the compressor vendor's scope. In fact the package datasheet mentions the maximum outlet temperature from the compressor package generaly as 50-55°C. The reason for restricting to a maximum of 50-55°C is due to the fact that any air temperature at or above 60°C would require the air system to be insulated for personnel protection.

When I mention that the outlet temperature from the compressor package should be a maximum of 50-55°C, the compressor package vendor ensures that his design provides the outlet temperature that does not exceed these values by providing the required cooling in terms of inter and after-coolers.

As a general rule most air compressor package vendors say that they can cool the air to 10 to 15°C above the maximum ambient air temperature at the inlet of the compressor. This is normally based on the inter and after-cooler being air-cooled heat exchangers.

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 07 March 2012 - 03:20 AM.

[S.AHMAD]

Dear MM
1. For determining the design temperature of piping and equipment (e.g. pressure vessel), I normally used the worst case scenario. In this case, the worst case scenario for temperature would be due to failure of after-cooler (e.g. failure of utility). Please take note that, the strength of CS will be the same up to 204C (the allowable stress remains constant). Therefore, as far as mechanical strength of CS, it makes no difference between 50C and 200C. However, the properties of process fluid (e.g. air) will change.
2. The method for determining the design pressure is based on the company guide that I am currently work with. I am not sure whether it is referred to any International Std. Other companies may be using different approach. What ever method that you use, the system will be protected by the PSV. If you want to install the PSV at the air receiver, ensure that no valves between the compressor and the receiver.

[mm217]

Dear Ankur;

That was really good advice!

I try to convince my PSL.

[mm217]

Dear AHMAD;

I agree with you. Thanks a lot for your help.