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T-Type Strainers, also known as 'Bath Tub' strainers, consist of an equal tee fitting and weld neck flange. T-Type strainers, therefore, have their basket dimensions and subsequent open area limited because the inside dimensions of the housing are restricted by this construction.
T-Type strainers were developed originally by North Sea Oil companies for use in commissioning applications. They were used in preference to conical strainers because it was possible to remove the screen for cleaning during the commissioning process without disassembling the line. The time saved during commissioning more than compensated for the additional financial outlay for the T-Type strainers compared to conical strainers and quicker commissioning processes were achievable enabling a reduction in total Project Expenditure.
After commissioning it was common for the basket to be removed and only installed again at re-commissioning cycles at annual pipe line maintenance intervals.
Within the Petrochemical Industry over the last decade, there has been effort to drive down costs and we have found that many contractors are specifying T-Type strainers for use in permanent applications. This is because they are the cheapest form of strainer that can be purchased with its own housing.
Of course, for use in permanent applications there can be many problems if there is any significant dirt loading. This is because the Filter Area of the basket of a T-Type strainer is approximately 250% of the inlet area. Therefore, if you use a perforated plate (for example Dia. 3mm hole perforated plate which has 40% open area) the resultant open area ratio of the T-Type strainer basket is only 40% x 250% = 100% of the pipe bore area. Therefore, as soon as the strainer starts to get blocked, the open area is immediately reduced below the inlet area and the pressure loss starts to increase quickly.
The problem is compounded if you line the basket with mesh, as the mesh further reduces the open area of the basket. For example, if you use a 10mm Square Hole Perforated Plate (with 70% Open Area) to support an 80 mesh wire cloth (with 34% Open Area) the resultant open area % of the basket is 70% x 34% = 23.8%. This leaves a reduced open area ratio of the basket from 250% filter area to 59.5% Open Area relative to the pipe bore. Therefore there is a high pressure loss before the basket even starts to get blocked and there is very little open filter area to hold solids.
For a permanent application with fine filtration such as 80 mesh, we would recommend the use of vertical basket filters, which would easily provide the required filter area, (open area 200% as standard) and large dirt holding capacity to give satisfactory performance in service.
In all cases T-Type strainer baskets should not operate with pressure loss above 1.0 Bar dirty, as the stresses on the basket can lead to failure. Also the baskets should be replaced, if possible, after every year of installation and operation to prevent fatigue occurring and consequential basket failure. If a basket is inspected after 1 year and shows no sign of fatigue damage after DPI, then it can be operated up to a maximum period of 2 years, but must be replaced every 2 years of operation without fail.
For a permanent application we would always recommend the use of vertical basket filters, which would easily provide a filter open area of at least 200% as standard, thus reducing forces applied to the baskets in operation and minimising the risk of failure.
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 +44 (0)1527-831644, 575211, 831256
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