Plant Instrument Air System – Useful Design Tips

by S. Zaheer Akhtar

The equipment associated with the Instrument air system as used in the industry, generally consists of an air compressor, air dryer and an air receiver fitted with a liquid drain trap. The instrument air system is utilized by various plant instrumentation, some of which may have a critical role in plant operation and safety. Therefore, the equipment should be properly sized and capable of producing the required air quality.

This article provides an overview of some of the key points and calculations associated with the instrument air system, which can assist the design engineer in his task.

Air Quality Specification:

The Specification for Instrument air quality is governed by ANSI/ISA-7.0.01 “Quality Standard for Instrument Air”. This specification stipulates the following:

  1. Pressure Dew Point: “The pressure dew point as measured at the dryer outlet shall be at least 10C (18F) below the minimum temperature to which any part of the instrument air system is exposed. The pressure dew point shall not exceed 4C (39F) at line pressure”.
  2. Particle Size: “A maximum 40 micrometer particle size in the instrument air system is acceptable for a majority of pneumatic devices”.
  3. Lubricant Content: “The lubricant content should be as close to zero as possible and under no circumstances shall it exceed one ppm w/w or v/v.”
  4. Contaminants: “Instrument air should be free of corrosive contaminants and hazardous gases which could be drawn into the instrument air supply”.

Calculation of Pressure Dew Point:

Note that the pressure dew point is the dew point at line pressure (example, -40F dew point at 100 psig.) As line pressure increases the dew point increases. Traditionally, regenerative desiccant air dryers for instrument air systems are capable of providing high level of moisture removal (usually -40F dew point but as low as -100F) over a wide range of air flow rates used in the industry.

Consider ambient conditions of 14.7 psia, 68F and 40 percent Relative Humidity. The dew point can be calculated as follows:

Sat. vapor pressure at 68F= 0.3389 psia (from steam tables)

Therefore moisture vapor pressure at 40% RH= 0.3389 x0.4=0.135 psia

From steam tables 0.135 psia is the saturation vapor pressure at 43F, therefore normal dew point is 43F

Now consider the same case when dew point (that is, pressure dew point) is evaluated at a higher pressure of 114.7 psia. In this case, the moisture vapor pressure evaluated above at 0.135 psia is multiplied by the pressure ratio, giving 0.135 x 114.7/14.7= 1.053 psia. From steam tables, this is the saturation pressure at approximately 104F.

Therefore the pressure dew point is 104F compared to normal dew point of 43F.

Regenerative Desiccant Air Dryers:

There are various types of air dryers such as the regenerative desiccant type, refrigerated type, deliquescent type, membrane type and point-of-use type dryer. Each type of dryer has its own limit on the air outlet dew point. Regenerative desiccant air dryers are commonly used in the process industry and are the most expensive.

The dryers work by adsorbing moisture on desiccant material such as alumina, silica gel molecular sieves. The desiccant material is contained in two packed towers which are alternately in service or being regenerated. The regeneration can be achieved by different methods, such as, using a purge of dry air from the operating tower or by using internal heaters or by an external heat source. Use of purge of dry air is a simple process with the discharge air purged to the atmosphere. However, use of purge air consumes about 15 to 20 percent of the compressed air capacity and is best utilized when there is sufficient excess air capacity.

The capacity of air dryers as provided by the vendor is generally in terms of “inlet scfm” at a service pressure of 100 psig and service temperature of 100F. At different service conditions, the inlet flow to the dryer needs to be corrected by multiplying by the pressure correction factor and the temperature correction factor.

If service pressure is higher than 100 psig (say 120 psig) the flow capacity of the dryer increases by the pressure ratio (120+14.7)/(100+14.7)=1.17. On the other hand, if service temperature is higher than 100F (say 115F), the flow capacity of the dryer decreases by the ratio of moisture saturation vapor pressure ratio at 100F and 115F (that is, 0.9492 psia/1.4711 psia=0.64).

Based on above, the corrected flow to a dryer rated for inlet of 100 scfm (at 100 psig/100F) and operating at 120 psig/115F, would be =100 x 1.17 x 0.64 = 75 scfm. Note that this is 25% less than the rated value of 100 scfm.

Desiccant Type Air Dryer Schematic – 1

Possibility of Mismatch between Compressed Air Supply and Demand:

The term “standard cubic feet per minute – scfm” should be used with caution due to variation in values used to represent standard pressure and standard temperature. In the compressed gas industry, standard conditions are taken as 14.5 psia, 68F and 0 percent relative humidity. Other variations for standard pressure/temperature conditions are 14.7 psia and 32F. As such, these variations can cause confusion and result in a mis-match between the compressed air supply and compressed air demand.

For example, consider an end-user demanding 10 scfm of air with the understanding that standard conditions are at 14.7 psia and 32F. Air density (ρ) at these condition of pressure and temperature is ρ=p/RT= 0.808 lbs/cu. ft. In other words the air demand is for 10 ft3/min *0.808 lbs/ft3= 8.08 lbs/min.

Now consider a reciprocating compressor supplying 10 scfm air at standard conditions of 14.5 psia, 68F. Air density (ρ) at these condition of pressure and temperature is ρ=p/RT= 0.742 lbs/cu. ft. In other words the air supply is providing 10 ft3/min *0.742 lbs/ft3= 7.42 lbs/min of air. This quantity of air is only 92% of the air demand of 8.08 lbs/min and therefore does not meet the requirement of the end user.

In case of a centrifugal compressor, the lower ambient pressure and higher ambient temperature at suction conditions leads to lower discharge pressure.

Note that in some cases, compressor capacity is stated in terms of Free Air Delivery (FAD) which is merely the discharge volumetric flow converted back to inlet conditions of the compressor.

Effect of Relative Humidity on Compressor Inlet Conditions:

As mentioned earlier, the compressed gas industry uses 14.5 psia, 68F and 0 percent relative humidity as the standard conditions. Now consider actual site conditions at 14.5 psia, 68F and 100% relative humidity. The moisture saturation vapor pressure at 68F is 0.339 psia, therefore the dry air pressure is reduced from 14.5 psia to 14.161 psia (14.5 psia-0.339 psia= 14.161 psia). In turn, this reduced pressure value at compressor suction decreases the mass flow capability of the reciprocating compressor (or decreases the discharge pressure in case of a centrifugal compressor).

Air receiver with Liquid Drain Trap and Balance Line – 2

Volume of the Air Receiver Tank:

The air receiver provides a storage volume of compressed air to be used when the compressor is off-line or when the air demand temporarily exceeds the compressor output. If the users require 100 psig in the air receiver and the compressor is set to provide 100 psig air to the receiver, then there is no hold-up or buffer. For the air receiver to be effective, it must therefore operate within a pressure band.

For example, assume that the demand for compressed air requires 100 psig pressure in the air receiver and the compressor is set to operate in a pressure band to load at 120 psig and unload at 130 psig. This means that in case the compressor is off-line, or if air demand increases, a storage volume corresponding to air receiver pressure ranging from 120 psig to 100 psig is always available. The air receiver volume can be calculated from the following equation which shows the time taken for the air receiver to drop from the higher pressure point to the lower pressure point within the operating pressure band:

t=V(p1-p2)/C*pa

Where,

t= time, mins

V= volume of air receiver, cu ft

p1= upper limit of air receiver operating band, psia

p2= lower limit of air receiver operating band, psia

C= net air consumption (scfm)

pa= atmospheric pressure (psia)

If air is supplied to the air receiver during the time interval being evaluated, then the value of C must be reduced by the rate of air supplied.

Air Compressor:

There are three types of air compressors generally used in the industry. These are as follows:

  1. Centrifugal Compressors
  2. Reciprocating Compressors
  3. Rotary Screw Compressors

Centrifugal compressors are cost effective in large sizes only, can provide oil-free air delivery and have the characteristic pressure curve with pressure decreasing as capacity increases.

The reciprocating compressors have effective multistep capacity control but has a high first cost with special foundations for vibrations and needs routine maintenance.

The rotary screw compressor is popular in instrument air service since it is a compact package at a relatively low first cost and provides oil-free air.

Compressor Control Strategy and Air Receiver Volume:

The rotary screw compressor capacity can be controlled by a variable speed drive. However for oil free compressors speed turndown is limited to about 50% of maximum speed depending upon adequacy of bearing lubrication at low speed and on compressor discharge temperature. Therefore during periods of low air consumption, the compressor will need to be unloaded with the discharge-to-suction bypass open (and if an over-run timer is fitted, it can stop the compressor in case it runs in unloaded condition for a pre-set period of time). With a large sized air receiver, the compressor will be unloaded for a longer period of time thus minimizing wear and tear associated with the compressor’s load/unload frequency or start/stop frequency. Therefore the air receiver should be sized accordingly keeping in view the associated wear/tear effect on the compressor and motor.

Moisture Drainage from Air Receivers:

The atmospheric humidity entering the air compressor ends up in the air receiver which is usually at a temperature below the dew point of the compressed air. Note that the pressure dew point is higher than the atmospheric dew point resulting in water accumulation at the bottom of the air receiver. This water is usually drained out through a liquid drain trap which drains the water while preventing escape of compressed air.

The balance line allows air which has entered the trap to be discharged back to the receiver. Without the balance line, air binding can occur in the liquid trap.

The size of the liquid trap depends on the differential pressure across the trap and the required discharge flow rate. The required discharge flow rate can be computed as follows:

Assume ambient air is at 14.5 psia, 70F and 70% relative humidity:

Saturation vapor pressure at 70F = 0.363 psia (from steam tables)

Vapor pressure at 70% Relative Humidity = 0.363 x 0.7 = 0.25 psia

Vapor pressure of dry air=14.5-0.25 = 14.25 psia

Mol fraction of water vapor=0.25/14.5=0.017

Assuming compressor inlet capacity of 500 scfm= 500/379.5=1.317 moles/min

Water vapor in compressor inlet= 1.315 x 0.017 =0.022 moles/min

Now (0.022 moles/min) x 18 x (60/8.338) = 2.89 gallons per hour. This is the amount of water to be discharged from the liquid trap at the bottom of the air receiver.

Conclusion:

The demand for compressed air may vary significantly at the plant. Therefore, the compressed air system must be designed such that that all components in the system (compressor, air dryer, air receiver and drain) are able to cope with the variation in demand. In this context, the design tips provided in this paper may help the system designer.

Author

S. Zaheer Akhtar, P.E. is Principal Engineer at Bechtel Corporation.

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