Complete Guide to Sanitary Butterfly Valves: Installation, Welding, and What We Learned from a Customer Seal Failure
By Zhang Wei, Senior Valve Engineer – OMSS Wenzhou Facility
Butterfly valves are one of the most common valve types in sanitary processing. They are simple, reliable, and cost-effective. But contractors who install them often run into problems that could be avoided with a few basic precautions. In this guide, we cover how sanitary butterfly valves work, how to install them correctly, and what we learned from a real customer case where improper welding destroyed four seal rings.
The Customer Case That Taught Us Something
A customer sent us photos of seal rings taken from four of our sanitary clamp butterfly valves. The seals were blackened, brittle, and in one case partially melted. The customer had ordered 12 valves for a CIP return line in a dairy plant. They had not yet put them into service. When they opened the valve bodies for inspection, they found the damaged seals.
Our first reaction, looking at the photos, was that this was not a seal material defect. The damage pattern was wrong for chemical attack or material fatigue. The seals had never seen process fluid. The charring had to have come from heat exposure during installation. Specifically, welding heat.
We asked the customer to send two of the affected valves back to our facility in Wenzhou, China. Before they arrived, we pulled the batch records for the seal ring material. The EPDM batch had passed all incoming QC tests: hardness (Shore A within spec), compression set (under 30%), and immersion test (168 hours, no significant swelling). The material was good.
When the valves arrived, our engineering team – led by Zhang Wei – inspected them. The findings were conclusive: the seal rings showed a charring pattern consistent with conducted heat. One valve disc had a measurable bow of approximately 0.15 mm in the center. The damaged seals came from valves that had been welded into the line with the discs in the closed position.
(Source: Customer return analysis 2026-Q1.)
Seal Ring Damage Mechanism: Three Stages
The heat damage to EPDM seal rings follows a three-stage process:
Stage 1 – Softening (invisible): The seal ring surface softens as temperature rises above 120 degree C. No visible change. The seal may still function initially.
Stage 2 – Hardening (invisible): Continued heat exposure causes the elastomer to crosslink excessively. The material becomes harder and loses elasticity. Performance degrades but may not be immediately noticeable.
Stage 3 – Carbonization (visible): Above 250 degree C, the seal ring turns black, brittle, and may crack or crumble. This is the stage our customer saw when they opened the valves.
Stages 1 and 2 are not visible to the naked eye. A seal ring that looks normal can already be damaged. This is why following the correct welding procedure matters, even if the seal appears fine after installation.
Why Opening the Disc Fully Is the Solution
When the butterfly valve disc is in the fully open position (parallel to the pipe), the disc edge has zero contact with the seal ring. Heat from welding transfers through the valve body and disc, but because there is no contact point, the heat path to the seal ring is interrupted. The seal ring only receives conducted heat from one side through the body groove, which is significantly less than when the disc is closed and pressing directly against the seal.
In the closed position, the disc edge presses firmly against the seal ring. Heat conducted through the disc transfers directly into the seal at the contact interface. This concentrated heat input is what causes the three-stage damage, often within a single welding pass.
Common Installation Mistake That Damages the Seal
The most frequent problem we see is a leaking valve after installation. The cause is usually the same: the installer welded with the butterfly disc in the closed or partially open position.
The heat from welding (TIG welding runs at temperatures well above the melting point of stainless steel) travels through the valve body and hits the rubber or PTFE seal ring inside. Even a short welding cycle can deform or carbonize the seal material. The result is a valve that leaks around the disc within days or weeks of going into service.
Correct Welding Procedure: 8 Steps
1. Inspection – Verify valve body, disc, and seal ring condition. Check that the valve matches the pipe specification.
2. Pipe preparation – Cut and bevel pipe ends per welding procedure specification. Clean the weld area. Verify alignment without the valve.
3. Positioning – Position the valve body between pipe ends. Do not force alignment. Check that the valve is centered and level.
4. Open the disc fully – Rotate the disc to the fully open position. Verify zero contact between the disc edge and the seal ring. This step is critical.
5. Argon purge and weld – Set up internal argon purge (99.99% Ar, 5-15 L/min). Weld the first end using TIG or orbital welding. Maintain purge flow throughout and for 10 seconds after arc extinction.
6. Sequential welding – Let the first weld cool to room temperature. Then weld the second end. This prevents heat buildup in the valve body.
7. Post-weld treatment – Passivation with pickling paste (20-30 minutes), internal flush, borescope inspection of internal weld bead.
8. Functional test – Reassemble the valve. Conduct a seat leakage test at 1.1x rated pressure. Run two CIP cycles before putting into service.
TIG Welding Parameters
Welding method: Automatic orbital or Manual TIG | Shielding gas: Argon 99.99% | Internal purge: Argon 99.99% | Purge flow: 5-15 L/min | Torch flow: 8-12 L/min | Electrode: 1.6-2.0 mm tungsten | Filler wire: ER316L | Current: 50-120 A (adjust by wall thickness)
Installation Guidelines
Key prohibitions:
Do not close the disc during welding – Heat expansion can permanently deform the seal ring. Keep the disc in the fully open position throughout.
Do not weld with the seal ring installed – Remove it before welding. For one-piece liner designs, wrap the valve body with a wet cloth during welding.
Do not force alignment – Correct pipe alignment first. Forced alignment creates residual stress that causes uneven disc wear.
Spacing requirements: Straight pipe before and after the valve: at least 3D. Distance to adjacent valves: at least 5D.
Support: Do not use the valve body as a pipe support. Install pipe supports 200-300 mm on each side of the sanitary valve.
Common Installation Errors
Welding with disc closed – Seal destroyed by heat. Valve replacement needed.
No argon purge – Weld root oxidizes. Contamination risk.
Forcing pipe alignment – Gradual leakage over time.
No passivation – Weld zone rusts. Sanitary audit failure.
Using valve body as pipe support – Seal loses fit. Valve leaks.
No straight pipe section – Flow turbulence shortens seal life.
These errors account for most field failures we see. The first two are the most expensive because they often require valve replacement.
Selecting the Right Butterfly Valve
End connection: Weld-end gives the most hygienic installation. Clamp-end gives flexibility and easier maintenance. Thread-end is for non-sanitary sections.
Body material: SS304 for water. SS316L for pharmaceutical and aggressive CIP chemicals.
Seal material: EPDM for food and dairy. Silicone for high temperature and SIP. PTFE for chemical resistance but requires higher torque.
Actuator mounting: Check for ISO 5211 mounting pad if automation is planned later.
FAQ
Q: Can I weld a butterfly valve with the disc in the closed position? A: No. The seal ring will be damaged within one welding pass. Always open the disc fully.
Q: Can I partially open the disc to reduce heat exposure? A: No. Partial opening still maintains contact between the disc edge and the seal ring. Only the fully open position eliminates contact completely.
Q: How do I know if a seal ring has been heat damaged? A: Visible signs: brittle, dark brown or black, glazed surface. Invisible signs (Stage 1-2): hardness increase of 5+ points Shore A. If in doubt, replace the seal ring after welding.
Q: What seal material is best for SIP service? A: Silicone handles up to 205 degree C. EPDM is suitable up to 130 degree C. For regular SIP at 121-134 degree C, use silicone.
Q: How often should the seal ring be replaced? A: Every 12 months for standard food and dairy. Every 6 months for pharmaceutical or frequent SIP.
Q: Can MIG welding be used instead of TIG? A: MIG produces higher heat input and spatter. For sanitary butterfly valve installation, TIG or orbital welding is recommended. MIG may be acceptable for utility connections only.
Technical Review
Zhang Wei, Senior Valve Engineer – 15 years in sanitary valve manufacturing. Led the seal failure investigation described in this article.
onemo@onemoss.com
Data Sources
OMSS butterfly valve assembly procedure QA-BFV-2025
OMSS QC procedure manual, welding section Q2-2025
Seal material batch QC records QA-SEAL-2025-Q1
Customer return analysis: seal failure investigation 2026-Q1
This article was updated based on a real customer case handled by our engineering team in 2026.
