What a drain cap does in a filter plate, how the wedge-wire screen cylinder is built, the cap and handle configurations available, and the data we need to quote one.
← Back to Stainless Steel Drain CapA drain cap — also called a filter nozzle, water cap or strainer nozzle — is the distribution element screwed into the filter plate at the bottom of a filter cell. One cap is one flow path between the plenum below the plate and the media bed above it. A stainless steel drain cap is the metal version of the same element: instead of a moulded polymer head it uses a wedge-wire screen cylinder, a cap, a threaded stem and a nut, so the flow openings, the strength and the material grade all come from metal parts.
In service, water passes down through the bed and enters each cap through the slots of the screen cylinder, then leaves through the stem and the plenum to the underdrain. The slots are sized against the fine end of the media grading so that the media — sand, anthracite, activated carbon, resin or a multi-media blend — is held back and cannot be lost to the clear water side. In backwash the flow reverses: water, and in a combined air-and-water backwash system air as well, is pushed up through the same caps and spread across the plate area. It is this even spread, not the individual cap, that lifts and cleans the whole bed instead of only the part sitting above an open tapping.
Two properties decide whether the cell behaves as designed. The first is the slot opening, which has to retain the media while passing the flow within the head loss allowance of the backwash pump and blower. The second is even distribution across the plate, which follows from the number and spacing of the caps and from the pressure loss across the plate assembly. A metal cap is chosen when the duty calls for it — for example where a polymer head is not acceptable for the medium or the service temperature, where the specification names a metal nozzle, or where repeated backwash loads have to be carried by a metal body.
The cap is a small assembly rather than a single moulding. Its main parts are the screen cylinder, the cap that closes the top of the cylinder, the stem that carries the flow through the plate, and the fixing set — a hex nut, a flange and a sealing washer — that clamps the unit to the plate.
The slot width is set against the fine end of the media grading so the bed cannot pass into the underdrain. The total open area of the cap then has to pass the service flow and the backwash flow within the head loss allowance of the system. Both figures depend on the media and the duty rather than on the cap alone, so the slot width is confirmed with the quotation, and the number and spacing of caps across the plate are fixed at the same time — it is the plate layout, not the individual cap, that governs how evenly water and air are distributed.
The cap form and the stem length are the two choices that follow from the installation rather than from the flow duty.
| Item | Options | Set by |
|---|---|---|
| Cap form | Dome cap · Flat cap | The plate layout and the space available above the plate |
| Handle type | Short handle · Long handle | Plate thickness and depth of the media bed |
| Mounting | Threaded stem with hex nut, flange and sealing washer | The existing tapping and the plate face |
| Screen cylinder | Wedge-wire slotted cylinder | Slot width from the media grading; diameter from the plate spacing |
| Material | Stainless steel | Grade confirmed against the medium, temperature and duty |
| Other arrangements | On request | Send a sample cap or a dimensioned drawing to match |
Cap form, stem length and thread are not interchangeable on a built plate: a long handle cannot be shortened on site and a different thread does not fit an existing tapping. Confirming these three details before the order is what keeps a replacement cap a drop-in item.
The cap is made in stainless steel, but the grade is a selection rather than a fixed figure. Grade choice follows the medium being filtered and backwashed through the cap — its chloride content, pH and temperature — and the duty the plate has to take. Because those conditions differ between a potable water filter, an activated carbon adsorber, a demineralisation unit and an industrial effluent filter, the grade is confirmed for each order instead of being published as one standard.
Where the water chemistry or the plant specification calls for a particular grade, tell us which one; where it does not, send the water analysis and we will confirm a grade with the offer. Material confirmation is issued with the quotation so the grade can be checked against the project specification before the order is placed.
| Parameter | Value | Remark |
|---|---|---|
| Body material | Stainless steel | Grade confirmed against the medium and duty |
| Screen | Wedge-wire slotted cylinder | Continuous V-profile slot |
| Cap form | Dome cap · Flat cap | Other forms on request |
| Handle options | Short handle · Long handle | Selected against the plate thickness |
| Mounting | Threaded stem with hex nut, flange and sealing washer | Thread confirmed per plate |
| Slot width | On request | Set by the media grading to be retained |
| Screen cylinder diameter | On request | Linked to the plate layout spacing |
| Overall length | On request | Set by the plate thickness |
| Thread size | On request | Matched to the existing tapping |
| Sealing arrangement | On request | Confirm the sealing detail for the plate |
| Flow per cap | On request | Confirmed with the flow rate and head loss allowance |
| Backwash water rate | On request | Confirmed per project |
| Backwash air rate | On request | Confirmed per project |
| Service temperature | On request | Confirm the medium and operating temperature |
| Service life | On request | Depends on the medium and operating conditions |
| MOQ | On request | Quoted against the quantity per filter cell |
| Price | On request | No price is published on this page |
| Item | Detail required |
|---|---|
| Filter type | Gravity, pressure, biological aerated filter, activated carbon or ion exchange |
| Plate | Plate type and thickness, and whether it is cast in place or already finished |
| Tapping | Thread size and tapping detail, or a sample cap to match |
| Media | Media type and grading, so the slot width can be set to retain it |
| Duty | Service flow rate, backwash water rate and backwash air rate |
| Water chemistry | Chloride content, pH and service temperature, so the grade can be fixed |
| Quantity | Number of caps per cell, number of cells, and the quantity of spare units |
| Commercial | Destination port and the transport mode |
Stainless steel drain caps suit media-retaining and distribution duties in gravity and pressure filters, and are used where the plate already exists or a metal nozzle is named in the specification.
Drain caps are made to the plate and to the media grading, so the dimensions that matter — slot width, screen cylinder diameter, overall length, thread and cap form — are set per order and are shown here as On request rather than as a fixed catalogue table. The photographs on the product page show production runs of the cap in dome-cap and flat-cap forms and in short-handle and long-handle lengths; they are not a size chart.
No price is published. Send the plate and media data listed above and we will return a quotation with a dimensioned drawing and the material grade for the duty. Compiled from our own production data and general filtration practice; where a project specification names a slot width, thread, length or grade, that specification governs.