8 replies to this topic

[arkae]

Hi every one,

I have a question about compressed air system discharge pressure.

First of all i would like to explain our present compressed air system

Compressor(2 x 100%) – Air Reciever1 – Air Dryer – Air Receiver2 – Instrument Air Distribution system

                                                               – Service Air Distribution System

I have 2 questions

1. Compressor is directly connected to air receiver1. After Air Receiver1, comressed air branches off to Instrument air distribution sytem and service air disribution system. A pressure transmitter present within the compressor senses the discharge pressure and then loads and unloads accordingly. The loading set point is 7 barg and unloading setpoint is 8 barg. As far as i understand, as the air in the air receiver1 is utilized, the pressure decreases and the pressure transmitter within the compressor senses low pressure and compressor loads. My question is how air consumption and pressure are related?

 

2. There is a pressure transmitter after Air Dryer and before Air Receiver2. Based on the setpoint, a on/off pneumatic valve present in the Serice Air Distribution is operated. If the Instrument air header pressure decreases(< 6.5 bag), the on/off valve present in service air line will be closed to maintain Instrument Air header pressure. But again, how air consumption is related to pressure? But my actual question is –“ Instead of on/off valve our customer is asking for control valve, so that service air will be available may be with less pressure at all times. But now based on Instrument air header pressure please tell me how to size the control valve in the service air line.

[chemsac2]

arkae,

 

Pressure is always that drives the flow. Imagine instrument air receiver pressure as consisting certain number of moles.

 

As you transfer moles from IA receiver to user, moles in IA receiver decrease and hence pressure there.

 

Compressor adds moles to IA receiver increasing its pressure.

 

I would recommend retaining ON-OFF valve on service air line as that ensures IA supply is always available.

 

Control valve with pressure control of IA system can also work. This would be analogous to compressor sending moles to system, control valve on service air system sending only those many moles of air that maintain IA pressure in header.

 

Regards,

 

Sachin

[S.AHMAD]

arkae

1. you need to return back to basic of gas law: PV = nRT and mass balance: mass in - mass out = accumulation.

2. You can assume that V, R and T are constants and n varies due to consumption of instrument air and service air.

3. As n decreases consumed by instrument/service air , so does P, the pressure. Once the compressor running, n increases so does the pressure.

[arkae]

Thank you very much for clearing my basics.

 

But with respect to sizing of control valve in service air line, please help me in what data should be provided to control valve vendor.

 

My question is how to decide the outlet pressure or outlet flow of the control valve that can maintain the instrument air header pressure(>=6.5 barg).

 

 

Thanks in advance.

[Bodhisatya]

As of now what is the flow distribution pattern.What is the accumulated discharge flow (in Nm3/hr) of two Air Compressors,and what is the distribution pattern in terms of flow of wet air post Air Receiver 1.How much is going to Dryer and how much to Plant Air system.  

 

Bodhisatya.

[arkae]

Each compressor capacity is 900 Nm3/hr. And only one compressor will be working and other will be standby.

 

Service air requirement is intermittent. Total 11 service air points are present. Considering 30 Nm3/hr for each service air point, the maximum air service air consumption will be 330 nm3/hr.

 

The compressor is sized considering only 10% of continuous usage of service air i.e continuous usage of service air will be 0.1*11*30 =33 nm3/hr.

 

And the rest of the air will be utilized for Instrument air.

[chemsac2]

arkae,

 

Size control valve for following conditions:

 

Maximum Cv case (maximum flow, minimum DP):

Flow = 330 Nm3/hr

Pin = Minimum header pressure

Pout = Atmospheric pressure + service air point pressure drop + service header line pressure drop for 330 Nm3/hr

 

Minimum Cv case (minimum flow, maximum DP):

Flow = 33 Nm3/hr

Pin = Normal header pressure 

Pout = Atmospheric pressure + service air point pressure drop + service header line pressure drop for 33 Nm3/hr

 

This pressure control valve will have its sensing point upstream side i.e. compressor side. In order to maintain header pressure, control valve opening and hence service air flow would be throttled.

 

Regards,

 

Sachin

[Bodhisatya]

 

 

This pressure control valve will have its sensing point upstream side i.e. compressor side. In order to maintain header pressure, control valve opening and hence service air flow would be throttled.

 

 

We already have a Pressure Control at Compressor Discharge which loads and Unloads the Compressor as per Post 1.

 

For me the tapping of PIC should be from Air Receiver 2 which will directly sense the consumption pattern of Instrument Air.So if this pressure falls below 6.5 ,the valve will act on the Wet Air supply header and push more air to Air Dryer to maintain constant header pressure. 

Correct me if my understanding is wrong 

 

Bodhisatya.

[arkae]

Thankyou all for your reply.