This page provides an introduction and some general guidelines for strainers to support the specification and selection of filtration equipment. It is not intended to be a definitive work and does not cover specialist strainers. Instead it is focused on the most common strainers used in today’s process applications in the oil and gas industry.
Strainers differ from filters in that a strainer typically provides a single barrier to the process stream and has a fixed opening (eg. 0.5mm perforated holes). Filters are normally constructed from a fibrous material, which creates paths for the process stream to flow through of varying, random apertures. The debris is retained at various points in the material and the filter element eventually becomes loaded and has to be replaced. This article is aimed specifically at strainers.
It is always recommended to seek expert advice from the respective filter and strainer specialist for each application.
The following information is fundamental to ensure the correct specification of strainer equipment for its intended purpose.
The common purpose of a strainer is to remove debris from a process stream. Strainer elements are available with a range of filtration sizes from as large as 10mm perforations down to 25 micron (0.025mm) fine woven wire mesh. Correct selection of the filtration size is very important and should relate to the maximum particle size that downstream equipment (eg. pumps) is capable of handling.
If the filtration size is too large then escaping debris could seriously affect the performance of downstream equipment. A filtration size that is too small could result in increased maintenance to clean the strainer and/or increased pressure drop, which can be detrimental to the downstream process.
|Mesh Size||Micron Rating||Gap Size mm||Gap Size inch|
|20 Mesh||740 microns||0.74 mm||0.030 in|
|30 Mesh||500 microns||0.50 mm||0.020 in|
|40 Mesh||400 microns||0.40 mm||0.015 in|
|60 Mesh||250 microns||0.25 mm||0.010 in|
|80 Mesh||200 microns||0.20 mm||0.008 in|
|100 Mesh||150 microns||0.15 mm||0.006 in|
|120 Mesh||125 microns||0.125 mm||0.005 in|
|150 Mesh||100 microns||0.10 mm||0.0039 in|
|200 Mesh||76 microns||0.076 mm||0.0030 in|
|300 Mesh||50 microns||0.05 mm||0.0020 in|
Note: Gap size is subject to mesh wire thickness. Therefore data is indicative only.
It is not always the case that the strainer size should be equal to the line size. The strainer size should be determined from the potential dirt volume that the unit expects to retain and the maximum allowable pressure drop for the strainer. An undersized strainer will result in increased maintenance required to continually clean the elements. In the worse instances it can create pressure drop problems in a process stream.
Although it is very difficult to receive realistic information on the potential solids content of a stream it is crucial information when considering the sizing of the strainer body. The element should be sized to adequately cater for the potential solids loading expected from the process stream. Where the solids loading is expected to be particularly high, duplex (2 x 100%) strainers, Hellan type or automatic backwash strainers should be used to make the process of element cleaning quicker and easier. An undersized strainer will ultimately lead to process problems.
Downstream equipment such as pumps that require protection by strainers are normally also dependent upon a minimum head pressure in order to function correctly. It is therefore important that the maximum allowable pressure drop for the strainer is determined to ensure that the design of the strainer will not present problems to downstream process equipment.
As with all piping equipment it is important to specify the design pressure and temperature range. Although design temperatures and pressures are covered in piping classes, these details generally cover a range of potential applications and do not necessarily address the specific requirements of individual process lines. It is important for each strainer that the specific design conditions are provided.
Strainer materials should reflect the materials specified for the pipeline. This information is provided in the relevant piping class which normally also provides details on the relevant Material Data Sheet (MDS). It is standard practice for the MDS requirements to cover pressure retaining parts such as the strainer body and cover, but not items such as the strainer element. The element material should be of similar, or better, material standard to the body.
Strainers fall into the category of equipment that is covered by the Pressure Equipment Directive (PED). Equipment may be subject to CE Marking requirements in line with the guidelines presented by the PED. The fluid and size/volume of the strainer will determine as to whether CE Marking is required or not.
Automatic backwash strainers must also comply with other directives including the Machinery Directive (522), FEA-M and since 2003 it is mandatory for any electrical equipment operating in hazardous areas to comply to the ATEX Directive. CE Marking requirements are covered as necessary in accordance with the applicable directives.
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