Product Overview

Spirax Sarco DP143 pilot valve seal assembly is an original bellows-type replacement seal engineered for pilot-operated pressure reducing valves. It isolates the pilot valve mechanism from downstream pressure chambers, preventing process fluid bypass and maintaining steady secondary pressure in steam, air, and gas systems.

Designated as Item C in the manufacturer spare parts identification list, this seal assembly provides universal interchangeability across the full spectrum of DP valve sizes, covering DN15LC, DN15, DN20, DN25, DN32, DN40, DN50, and DN80 bodies. It installs into the pilot valve block over the pilot valve plunger, forming a flexible hermetic barrier that protects the pilot diaphragm mechanism during continuous pressure modulation.

Key Features

• Universal size interchangeability: one standardized assembly services all host valve body sizes from DN15LC through DN80 (marked ‘a’ across all size columns in interchangeability tables).
• Metallic bellows containment: flexible bellows structure isolates the pilot valve operating chamber without binding the actuation stroke.
• High-torque mechanical mounting: screws directly into the pilot valve housing with a specified tightening torque of 115 N m using a standard 24 mm A/F socket.
• Broad host compatibility: fully operational across cast steel DP143, DP143G, DP143H, and DP143Y models, as well as stainless steel DP163, DP163G, and DP163Y derivatives.

Technical Specifications

Parameter	Specification
Component Name	Pilot valve seal assembly (Bellows seal assembly)
Spare Part Reference	Item C (ref. 31 in IM-P006-07 / ref. 12 in IM-P006-08)
Host Valve Series	DP143, DP143G, DP143H, DP143Y, DP163, DP163G, DP163Y
Compatible Host Sizes	DN15LC, DN15, DN20, DN25, DN32, DN40, DN50, DN80
Socket Tool Size	24 mm A/F
Assembly Tightening Torque	115 N m
Pilot Plunger Datum Gap	0.7 mm between plunger top and diaphragm recess straight edge
Assembled Clearance	Line-of-light clearance between bellows top and straight edge
Maximum Host Design Pressure	40 bar g (PN40 flange) / 18.9 bar g (ASME 150 flange)
Maximum Host Working Temperature	350 °C (DP143H), 300 °C (DP143), 250 °C (DP163), 120 °C (DP143G/DP163G)

Applications

• Steam pressure reducing stations equipped with Spirax Sarco DP143, DP143H, or DP163 valves.
• Compressed air and inert industrial gas distribution lines utilizing soft-seal DP143G and DP163G valves.
• Hospital autoclaves, sterilizers, and critical low-pressure systems operating DP143Y and DP163Y reducing valves.
• Scheduled 12-to-18-month overhaul programs requiring complete pilot section resealing.

Installation and Fitting Guidelines

Disassembly and Access

1. Isolate the reducing valve from upstream and downstream lines, venting line pressure to zero.
2. Release lock-nut (2) and rotate adjustment screw (1) counter-clockwise until the pressure spring is completely slack.
3. Slide out ‘C’ washer (3) and lift off spring housing cover (12/4).
4. Remove adjustment spring (6) and top spring plate (5).
5. Remove the 4 x M10 nuts (8/9), spring housing (4/8), bottom spring plate (7), and pilot diaphragms (9/10).
6. Disconnect the 6 mm stainless steel pipework by loosening the pipe union nuts.
7. Remove nuts (21/11) securing pilot valve housing (10/30) to the valve body, ensuring main valve spring (16/20) remains correctly positioned over main valve head (17/21).

Seal Removal and Replacement

1. Unscrew pilot valve seat assembly (13/14) with internal screen (14/14A) using a 27 mm A/F socket and remove plunger (11/13).
2. Unscrew bellows seal assembly (31/12) using a 24 mm A/F socket.
3. With bellows seal removed, install new pilot valve assembly (13/14) and tighten to 115 N m (use a bonded seal between pilot valve assembly and block on ‘G’ versions).
4. Insert plunger (11/13) from above. Place a straight edge across the diaphragm location recess and confirm a 0.7 mm gap exists between the plunger tip and the straight edge. If production tolerances cause excess length, grind or machine the top of the plunger to size, ensuring all sharp edges are deburred to prevent bellows puncturing.
5. Carefully place the bellows seal assembly over the plunger and tighten down with the 24 mm A/F socket to 115 N m.
6. Re-check with the straight edge while pressing lightly on the top of the bellows; confirm a slight line-of-light clearance exists between the straight edge and the top of the bellows.

Reassembly Notes

1. Clean gasket mating faces on both pilot block and body. Fit new body gasket (15/19).
2. For DP163 stainless steel models, apply a thin smear of PTFE-based grease to mating screw threads to prevent galling.
3. Secure pilot valve block with nuts (21/11) to the specified torque: M10 nuts (DN15LC to DN20) to 40 N m (50 N m on IM-P006-08); M12 nuts (DN25 to DN50) to 60 N m; M12 nuts (DN80) to 80 N m.
4. Reconnect the 6 mm stainless steel tubing, refit pilot diaphragms (9/10) in original orientation, secure spring housing nuts (8/9) to 50 N m, and re-commission following standard pressure setting procedures.