Design Standard for Pneumatic Systems | Instrumentation

1. Purpose – This article is about Design Standard for Pneumatic Systems for instrumentation system and all technical requirements for pneumatic systems specified for air separation units (ASU) and flammable gas plants designed by Instrument and Electrical Design.

2. Design Standard for Pneumatic Systems | Instrumentation

1. This standard applies to all circuits where instrument air is being used as a control or actuation medium.
2. This standard does not apply to process connections to instruments or hydraulic systems.

3. RELATED DOCUMENTS

Note: The following related documents offer further guidance on the design and installation of pneumatic systems.

Instrument Installation

British Standards Institute (BSI)

BSI BS 6739    Code of Practice for Instrumentation in Process Control Systems: Installation Design and Practice

4.  GENERAL

1. The normal pneumatic signal level shall be 0.2 to 1.0 bar g (alternatively 20 to 100 kPa or 3 to 15 psig, depending on the system of units specified for the plant).
2. The instrument air header system should be designed to operate in the range of 4 to 7 bar g whenever possible. Minimum instrument air pressures below 4 bar g may result in oversized actuators having to be used.
3. Pneumatic signal tubing shall be 6 mm (1/4 inch) to optimise speed of response. Other pneumatic tubing shall be 10 mm (3/8 inch) O.D. minimum for capacity reasons. Larger sizes may be required for large actuators.
4. The standard material for pneumatic lines is copper, with brass fittings. In areas subject to corrosive atmospheric contamination, particularly ammonia, stainless steel tube and fittings should be specified.
5. Compression fittings of an appropriate material for the tube shall be used (brass fittings for copper tube, stainless steel fittings for stainless steel tube). The preferred type and supplier of fittings is Swagelok twin ferrule. However, other compression fitting suppliers may be specified for particular projects to comply with the customer or site requirements.
6. Tube shall be adequately supported and protected from mechanical damage.
7. Non-metallic tubing shall not be used in exposed outdoor conditions. For pneumatic circuits only, nylon or PVC tubing is allowed inside panels and enclosures. Trunking shall be used to keep the installation tidy.

5.  INSTRUMENT AIR SYSTEMS

1. Each piece of equipment using air (air user) shall be provided with its own filter/regulator (air set).
2. Each air user shall have its own dedicated connection to the instrument air subheader or manifold, with an isolation valve.
3. Take-offs from headers and subheaders shall be from the top of the pipe.
4. At least one spare valved and capped connection to the subheader shall be provided for future modifications.
5. The sizing of air headers and subheaders in relation to the number users shall be according to the following table:

   Maximum Number of Users	Minimum Size Metric (mm)	Minimum Size Imperial (inches)
   5	15	1/2
   20	25	1
   50	40	1 to 1 1/2
   100	50	2
   200	80	3

6. Isolation valves between the main air header and subheaders shall be provided. These isolation valves shall generally be the same size as the subheader. Isolation valves for 15 mm (1/2 inch) subheaders shall be provided with 25 mm (1 inch) isolation valves that are mounted in the main header/subheader before reducing down to the 15 mm (1/2 inch) subheader.
7. The material specification of the subheader shall be the same as for the main instrument air header.
8. The pneumatic hook-up and air user installation downstream of the subheader shall be either stainless steel or copper/brass using O.D. tube and compression fittings.
9. Provision shall be made in the design and installation of the air system for water to self- drain to low points. Low points shall be provided with a drain valve and plug.
10. For instrument air produced by a compressor/drier system, the dew point of the air shall be at least 10°C below the lowest known ambient at the location (not the design minimum temperature). If the ambient is not known, the dew point shall be -40°C.

6.  SIZING OF INSTRUMENT AIR SYSTEMS

1      Instrument air requirements shall be based on the average consumptions provided in the following table.

Item	Average Unit Consumption
		Instrument Control Valve Switch Valve	0.75 Nm3/hr 1 Nm3/hr 0.5 Nm3/hr

2      A contingency of fifty percent shall be added to the figure calculated by using the table in paragraph 6.1, to allow for instruments missed from the count and for future additions.

3      Special consideration shall be given to areas of the plant containing sequencing valves, such as the molecular sieve switch valves, which use a lot of air for a short period, but very little for most of the time. Care shall be taken to ensure the instrument air pressure in the locality of these sequencing valves does not fall below the stated minimum supply pressure for the valves during switching operations, as this will slow down the operation of the valves.

Pressure Testing and Lay-up Procedures in Plant Piping and Pipelines