Installing a multi-stage filtration system as close as possible to the equipment powered by instrument air or instrument gas protects sensitive instruments, prolongs equipment service life and prevents unplanned shutdowns, as Phil Keating explains.
Field research reveals that as many as nine out of 10 valve failures are due to contaminants in the instrument air or instrument gas. These failures can be avoided with proper filtration, but a filter installed at the compressor or supply gas source is not enough. Many of the additional filter/regulators installed at point-of-use remove little water and no oil contamination.
Problems caused by poor air or gas quality can cause premature equipment failure and unplanned shutdowns. Filtering instrument air and gas prevents these problems, but having a filter set at the compressor may not be adequate.
Particulates, water and oil in the stream of instrument air or instrument gas can wreak havoc with downstream equipment. Here are just a few examples. Approximately 90 per cent of solenoid valve failures are caused by particulate contamination. Oil contamination can cause positioners to fail due to hydraulic lock between the spool and sleeve. Moisture and oil vapour degrade pump seal materials. If oil reaches the dryer media it will cause immediate and complete failure. In addition, oil or water can cause incorrect readings from analytical equipment. Any of these or other failures caused by inadequate filtration can result in costly and time-consuming maintenance and possible unplanned shutdown. The costs related to these failures can run into thousands or even millions of dollars.
A common approach to preventing these failures is the fitting of a single filter set at the outlet of the air compressor or supply source of the instrument gas. However, this does nothing to protect downstream equipment from contaminants introduced after the air or gas leaves the source. For example, pipework is often stored outside, where it is open to contamination, before being installed. Long pipelines from supply to instrumentation experience large variations in temperature, causing moisture to condense. Simple filter/regulators remove only little water and no oil to begin with, and the aggressiveness and impact of air system contaminants is magnified under compression. Finally, a 'one size fits all' filter set at the source is inadequate for most specialised equipment that requires finer filtration and oil removal.
There is growing awareness of the need for additional filtration at the point of use. However, most operators only install a single filter/regulator. Although this does help remove particulates, it provides little in the way of water removal and no oil removal.
Placing a multi-stage filtration system that includes a coalescing filter to remove oil vapour as close as possible to the equipment using instrument air or instrument gas protects sensitive instruments and keeps critical equipment operating as intended.
Determining filtration requirements
Industry standards and manufacturer’s recommendations highlight differences in requirements for clean air or gas, depending on the equipment being used.
There are some industry standards that provide guidance for designers and plant operators trying to determine adequate filtration levels. The International Society of Automation’s ISA-7.0.01-1996 Quality Standard for Instrument Air sets general standards for moisture content and particulate and oil contamination. This standard sets the maximum allowable oil content at 1 ppm. It requires removal of particles 40 µm or larger, with a caution that some devices require finer filtration. It also states that dew point should be 10°C below ambient temperature. Finally, ISA-7.0.01 states that there should be a sufficient quantity of air for the highest demand situations and that air intake should be free of contaminants.
This ISA standard sets a guideline, but it may be too general for certain pieces of equipment commonly used in oil and gas extraction, refining and hydrocarbon and chemical processing. A summary of a review of literature from multiple oil and gas equipment suppliers about their clean air/clean gas requirements is given here:
Summary of equipment suppliers’ clean air/clean gas requirements
Particulates Water Oil
Positioners 3-40 µm Dew point 10° below ambient temperature 1 ppm
Pumps 25 µm Dryer required N/A
Transmitters 5-40 µm Dryer required 1ppm
Analysers 1 µm Dew point 10° below ambient temperature 0.1 ppm
Actuators 25 µm Dew point 10° below ambient temperature 25 ppm
Clearly, different equipment requires different levels of filtration. The International Organisation for Standardisation’s document ISO 8573-1:2010 specifies classes of compressed air purity for particles, liquid water, moisture vapour and oil vapour. This standard provides a straightforward way to communicate an application’s specific requirements and to determine whether a filtration system will meet them.
Anatomy of good point-of-use filtration
A multi-stage filtration system should be designed to meet an application’s specific requirements and installed as close as possible to the equipment being powered. This is the best way to protect sensitive instruments, prolong equipment life and prevent unscheduled shutdowns.
Here are some properties to look for when specifying a point-of-use filtration system.
Reliable particle removal. Modern three-stage filtration systems use multiple filters to remove ever smaller particles. For example, the first stage removes particles 25µm or larger, exceeding the ISA 7.0.01 standard. The second filtration stage removes particles 5µm or larger. The third filtration stage removes sub-micron-sized particles. This multi-stage approach dramatically improves the air quality at each phase, prolonging the filter element life of the next stage.
High water extraction efficiency. Running instrument air or instrument gas through three filters in succession removes more than 95 per cent of liquid water.
High oil removal efficiency. General purpose filters do not remove oil contamination. The third stage of a three-stage filtration system is typically a coalescing filter for oil removal. The filter element can be designed to remove oil to 0.01 ppm, or 99.9 per cent of oil vapour.
Resistance to harsh operating environments. Offshore and on-shore oil and gas extraction, refineries, hydrocarbon and chemical processing plants all present harsh, corrosive environments. Filters used in these applications need to conform to National Association of Corrosion Engineers (NACE) standard NACE MR0175 for materials used in hydrogen sulphide (H2S) environments common in the petroleum and natural gas industries. Corrosion-resistant 316 stainless steel will resist sulphide stress cracking and discolouration. Seals made from high performance nitrile elastomers will operate across a wide temperature range and stand up to exposure to hydrogen sulphide.
Compatibility with instrument air and instrument gas. Basic filter/regulators designed for use with compressed air may fail prematurely under harsh conditions, particularly when used with instrument gas containing high levels of hydrogen sulphide. Three-stage filtration with components that meet the requirements listed above will prevent this problem.
Service life indicator. The life of a filter element is determined by the level of contamination in the air or gas, not by length of service. A visual indicator that signals when the filter element is so contaminated that flow is restricted makes it easy to know when to replace it. This ensures that the point-of-use air or gas is maintained at the expected pressure and filtration level.
Phil Keating is Business Development Manager - Energy Sector at Norgren. www.norgren.com