Obsolete Parts and Emery Cloth
A 4-inch RPZ at a food-processing plant has been in service 16 years. The manufacturer discontinued the model 5 years ago and the OEM repair kit is no longer available. A supply house sells a 'universal' kit that lists the model as compatible. The plant mechanic has already removed the No. 1 check, polished the seat with emery cloth, and replaced the disc with a generic O-ring of about the same size. The assembly now passes a field test, but the relief valve drips about a cup of water an hour and the No. 1 check differential reads 6 psi. The plant wants to keep it in service. This is the only containment assembly protecting a health hazard: a boiler chemical feed.
1. Is the repair acceptable? Set out the criteria you would use to decide repair versus replace.
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The repair as described is not acceptable, and the decision criteria point to replacement:
- Listing and approval. An RPZ is approved as a complete assembly. Universal kits and generic O-rings are not OEM parts; the assembly is no longer in the condition the approval covers, so the utility cannot rely on it for containment of a health hazard.
- Parts support. No OEM kit has been available for five years, which means every future repair will repeat this problem.
- Seat condition. The seat has been hand-polished with abrasive, which is itself a modification of a critical sealing surface.
- Age and service. Sixteen years on a chemical-feed system is past typical service life for elastomers and springs.
- Risk. The hazard downstream is a boiler chemical feed; a failure is a health-hazard event, not an inconvenience.
- Cost. A new 4-inch RPZ with a known parts stream costs less over the remaining life than repeated improvisation.
Permit the assembly to remain in service only as an interim measure with a written schedule for replacement, and only if it holds test. Here it does not: the relief drip means it is not holding.
2. Explain why grinding the seat with emery cloth and substituting a generic O-ring is a problem, and why a passing differential of 6 psi is not reassuring.
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Emery cloth removes the machined finish and changes the seat's geometry. The elastomer seals against a flat, lapped surface; an abraded surface leaves scratches that become leak paths, and abrasive grit embeds in the seat and disc, accelerating wear and contaminating downstream water. On a check valve, the seat is also the reference surface for the closing force, so material removed here changes how the disc seats and how the check behaves under reverse flow.
A generic O-ring is worse in a quieter way: it may differ in hardness, durometer, cross-section, and material compatibility. A ring that is 'about the same size' may seal at low differential and creep or extrude under a pressure reversal, and it may swell or harden in contact with the water chemistry at this plant.
The 6 psi differential proves only that the check closes firmly at the moment of the test. It says nothing about the seal's integrity under a real backflow event or about the relief drip, which indicates a seat or sensing problem that the differential measurement does not capture. A single passing number is not evidence that an assembly modified this way will hold when it matters.
3. What documentation and interim protection do you require, and what do you tell the plant about its compliance status?
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Interim protection: the plant is served by an unverified assembly, so require either a functioning replacement installed and tested within a defined window, or, if the utility permits, a temporary approved assembly, or continuous supervision of the connection with the boiler feed isolated. Boiler chemical feed must not be connected to the potable line without an air gap or a functioning RPZ in the interim.
Documentation to collect:
- The failed test report and the reason the assembly is out of service.
- A written description of the unauthorized repair, including who did it and when.
- The replacement assembly's make, model, size, serial number, and initial field test, plus the installer's certification.
- The permit and inspection record if the local program requires one for replacement.
- The retest and the new annual due date.
Tell the plant plainly: the assembly has been out of compliance since the relief began discharging, the repair did not restore an approved assembly, and continued service without correction exposes them to enforcement and to liability if contamination occurs. Give the deadline in writing and confirm receipt.