How to Choose a Bag Filter Housing Manufacturer
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Choosing a bag filter housing manufacturer comes down to seven checks, and price is the last of them. A bag filter housing is a welded pressure vessel with a seat that must seal a filter bag at every change; if the alloy is wrong, the welds are rough or the seat is a few millimeters off, it costs far more in bypass, corrosion and downtime than it saved on the invoice. Ask every manufacturer:
- Material proof: can they show the housing is 304 or 316 with a positive material identification (PMI) reading?
- Drawing capability: can they build to your drawing, or produce one for you to approve?
- Welds and finish: how are the welds made and finished, inside and out?
- Bag seat and seals: which bag ring style does the basket take, and which gasket materials are offered?
- Dimensions: will the delivered housing match the drawing?
- Range: do they cover standard #1 to #4 sizes and multi-bag vessels?
- Who you are talking to: a factory, or a trading company relaying your questions?
This is the manufacturer-selection chapter of our stainless steel filter housing buyer's guide, which covers the housing types themselves. Industry buying guides list flow rate, fluid compatibility, material of construction and ease of bag changes as the main selection factors (Filter Source, 2023). All four depend on the shop that built the vessel, which is why every question below is about the shop.
Question 1: Can you prove the housing is 304 or 316?
This comes first because it is where buyers get burned most: the two grades look identical, and the price gap tempts a careless supplier to ship 304 when 316 was ordered. The difference is 2 to 3% molybdenum in 316, which resists chloride pitting in seawater, brine and many chemicals; see 304 vs 316 for filter housings if you are still deciding.
The good answer is a PMI reading. A handheld X-ray fluorescence (XRF) analyzer reads chromium, nickel and molybdenum in seconds without damaging the part: molybdenum near 2 to 3% means 316, near zero means 304. XRF cannot read carbon, so separating 316 from 316L needs an optical emission spectrometer (OES) test. The methods are standardized under ASTM E1476 (XRF) and ASTM E1086 (OES), and API RP 578 describes how a material verification program is run (Projectmaterials, What is PMI testing).
Our 304 and 316 housings can be verified by spectrometer on your actual housing before it ships. With any manufacturer, remember the grade statement covers the housing body: ask for valve, gasket and fastener materials separately, in writing. A magnet proves nothing either way.
Question 2: Can you work from my drawing, or produce one?
The moment you need the inlet rotated, taller legs, a European flange standard instead of a US one, or the drain on the other side, you need a manufacturer that reads drawings and can produce one for you to approve. A supplier who only forwards your sketch will lose a dimension along the way.
The good answer is a dimensioned drawing back from the manufacturer showing port size and type, flange standard and class, nozzle orientation, overall and leg height, drain and vent positions, closure and finish, with the thread type in plain words. Our BSP ports are G (BSPP) parallel thread, sealed on a washer at the face, not NPT and not PT or R tapered thread. US lines normally need NPT; the UK, EU, Australia, New Zealand and Korea normally need G. How to tell NPT from BSP walks through it.
We work from customer drawings and draw our own; the custom bag and multi-cartridge housings page lists what to include so the first drawing is close to right.
Question 3: How are the welds made and finished?
A bag housing is welded at the shell seam, every nozzle, the basket seat and the legs. Each weld is a potential crevice for solids and a corrosion site if heat tint is left on. Ask whether the welds are continuous inside and out, and how each side is finished after welding: ground, pickled, passivated or polished.
The good answer names the inside and outside finish separately (mill 2B, brushed, mirror or electropolished), because they are often different and priced differently. A quote that says "polished" with no surfaces named is a red flag.
Question 4: Which bag ring does the seat take, and which gaskets are offered?
Two seals matter, and buyers usually ask about one. The lid gasket seals lid to shell; the bag seal is the bag's ring on the basket rim, pressed down so liquid cannot bypass the bag. A seat takes one ring style, steel ring or plastic snap ring, and they do not interchange, so tell the manufacturer which ring you stock before the seat is made.
For gaskets, the good answer is a choice of silicone, EPDM, FKM and PTFE with a recommendation for your fluid and temperature. For the lid, the options are a clamp closure (a band clamp holding the lid on, opened without tools), a swing bolt lid on larger vessels, or a bolted lid. A clamp closure is not the same as Tri-Clamp ports: one is how the lid comes off, the other is a sanitary ferrule connection for inlet and outlet. Name each separately on the quote.
Question 5: Will the delivered housing match the drawing?
An approved drawing is only useful if the vessel that arrives matches it. Ask how finished dimensions are checked before shipping and whether a measured record or photos can be sent. Our housings are delivered to the dimensions on the approved drawing; if a particular dimension matters, say so at order and we add it to the final check.
Question 6: Do they cover the standard sizes and multi-bag vessels?
Bag sizes are an industry standard, so the housing follows the bag. Covering the range means you can buy a #2 housing today and a six-bag vessel later from the same source, stocking one bag for both.
| Bag size | Nominal bag dimensions | Typical use | Where it is sold |
|---|---|---|---|
| #1 | 7 in x 16 in (178 x 410 mm) | Medium flow, limited headroom | Single bag housing #1 / #2 |
| #2 | 7 in x 32 in (178 x 810 mm) | Most industrial lines; about twice the bag area of a #1 | Single bag housing #1 / #2 |
| #3 | 4 in x 8 in (102 mm diameter) | Small flows, point of use | Small bag housing #3 / #4 |
| #4 | 4 in x 14 in (102 mm diameter) | Small flows, more area than #3 | Small bag housing #3 / #4 |
| Multi-bag, 2 to 8 x #2 | As #2, in parallel | Higher flow or longer change intervals | Multi-bag housing or a custom vessel |
Size follows flow and headroom; when one #2 housing changes too often, the next step is a multi-bag vessel or a duplex pair rather than a bigger single bag (PRM Filtration, #2 or #4 sizing). Felt, nylon mesh, stainless mesh and oil-absorbing bags for these housings are under filter bags.
Question 7: Factory or trading company?
Neither is wrong, but it changes who answers questions 1 to 5. A factory, or a supplier working directly with the factory floor, can read your drawing, move a nozzle, confirm a gasket and take a PMI reading without waiting for someone else; a trading company relays each question and loses a detail at each hop.
For our part: we have worked in stainless steel filtration for 7 to 8 years in close cooperation with the factory, which is why we quote as factory direct, accept a drawing or produce one, verify the grade by spectrometer and deliver to the drawing. The minimum order for a custom housing is one piece. The about page has the rest.
Manufacturer checklist: good answers and red flags
| What you ask | Good answer | Red flag |
|---|---|---|
| 304 or 316? | PMI (XRF) reading on the actual housing; body grade stated separately from valves and fasteners | "It is stainless", a magnet test, one grade claimed for all parts |
| Drawing | Dimensioned drawing returned for approval; thread spelled out as NPT or G (BSPP) parallel | Catalog picture only; "BSP" with no G or R stated |
| Welds and finish | Continuous welds; inside and outside finish each named | "Polished" with no surfaces named |
| Bag seat and seals | Ring style confirmed against your bags; silicone, EPDM, FKM or PTFE gaskets | Ring style unknown; one gasket only |
| Dimensions | Delivered to the approved drawing; key dimensions checked on request | No final check described |
| Who you deal with | Changes the drawing and answers weld and material questions directly | Every answer is "I will ask the factory" |
What should you send to get a real quote?
Send the fluid and its temperature, the flow rate, the bag size and ring style, the port size and type, the flange standard if flanged, the material grade, the closure, the finish inside and out, and a drawing or sketch if the layout is not standard. Stock #1 to #4 housings are in the bag filter housing collection; anything else starts on the custom housing page.
FAQ
How do I know a bag filter housing manufacturer really uses 316?
Ask for a PMI reading taken with a handheld XRF analyzer on your housing: molybdenum around 2 to 3% confirms 316, near zero means 304. Telling 316 from 316L needs an OES test. The reading covers the housing body; ask for valve and fastener materials separately.
What is the difference between a clamp closure and Tri-Clamp ports?
A clamp closure is a band clamp that holds the lid on the body so you can open the housing without tools. Tri-Clamp ports are sanitary ferrule connections for the inlet and outlet. A housing can have one, both or neither, so specify each separately.
Can a manufacturer build a bag filter housing to my own drawing?
Yes, if they fabricate rather than resell. We work from customer drawings and can produce one for approval; ports, flange standard, nozzle orientation, legs, drain and vent positions, closure, finish and material are all open. The minimum order for a custom build is one piece.
Should I ask for NPT or BSP threads?
Match your existing fittings. US lines are normally NPT tapered thread; the UK, EU, Australia, New Zealand and Korea normally use G (BSPP) parallel thread, which seals on a washer at the face. They do not interchange, so state the one you need on the order.
Ready to compare answers, not just prices?
Send us your drawing or a short description of the line and we will return a dimensioned drawing, the material statement and answers to all seven questions on one quote. Start at custom bag and multi-cartridge filter housings for anything beyond the stock sizes, or request a quote directly.