HSD diaphragm seal · Welded pipe saddle · On-line pressure isolation
In-Line Saddle Diaphragm Seal
Direct pressure measurement on 3-inch and larger process piping.
The Haygor HSD uses a welded saddle lower and bolted diaphragm assembly to isolate a gauge, transmitter, or switch at an on-line pipe measurement point. Published standard configurations cover 3-inch pipe and 4-inch or larger pipe, with standard 8-bolt ratings to 1,500 psi and replacement uppers for selected existing saddle designs.
- Welded saddle for an existing pipe wall
- Standard 3-in. and 4-in.-plus configurations
- Published standard rating to 1,500 psi
Quick answer
What is an in-line saddle diaphragm seal?
An in-line saddle diaphragm seal is a pressure-isolation assembly attached directly to the wall of a process pipe. A contoured lower saddle is welded over a prepared pipe opening, while a flexible diaphragm and sealed hydraulic fill system transmit process pressure to a gauge, transmitter, or switch without allowing the process media to enter the instrument.
The HSD is useful when an on-line measurement point is preferred over a conventional threaded or flanged branch connection. The saddle geometry brings the sensing diaphragm close to the flowing process and provides a serviceable bolted upper assembly. Haygor publishes complete 3-inch and 4-inch-or-larger configurations as well as replacement uppers for selected existing saddles.
Hydraulic pressure transfer
How the HSD saddle seal works
The HSD combines a pipe-contoured welded lower with a serviceable diaphragm upper and a sealed instrument fill system.
- 1
The lower saddle is fitted to the pipe
The correct 3-inch or 4-inch-and-larger lower is matched to the actual pipe outside diameter, material, wall, and prepared process opening.
- 2
The diaphragm isolates the instrument
Process pressure reaches the wetted diaphragm through the pipe opening, while the diaphragm prevents process media from entering the instrument fill path.
- 3
Fill fluid carries the pressure signal
A sealed fill fluid transfers diaphragm movement through a direct instrument connection or remote capillary arrangement.
- 4
The instrument reads isolated pressure
The gauge, transmitter, or switch responds to the transferred pressure while remaining separated from the process media.
Application fit
When an in-line saddle seal is a strong choice
The HSD is intended for on-line pipe pressure points where direct saddle mounting, a pipe-contoured lower, and a replaceable upper assembly align with the mechanical and process requirements.
Installation and process advantages
- Creates a direct pressure-isolation point on an existing process pipe
- Uses dedicated saddle geometry for 3-inch or 4-inch-and-larger pipe
- Brings the diaphragm close to the flowing process
- Allows the bolted upper to be serviced or replaced
- Supports specialty diaphragm and lower materials
Conditions that must be confirmed
- Pipe outside diameter, schedule, wall thickness, and material
- Design pressure, temperature, and piping-code requirements
- Saddle weld details, inspection, and installation access
- Media compatibility for diaphragm, lower, gasket, and fill fluid
- Bolt pattern and geometry for any replacement upper
Common industrial settings
Direct pipe-wall isolation for difficult pressure points
- Chemical processing
- Refining
- Petrochemical
- Pulp & paper
- Power & utilities
- Water & wastewater
Catalog specifications
HSD pipe-size, pressure, connection, replacement, and dimensional choices
The values below summarize the HSD selections published in Haygor's diaphragm seal ordering guide. Confirm the latest configuration and complete system limits with Haygor.
Dedicated small-pipe saddle
Published in standard 8-bolt and optional 6-bolt forms.
Larger-pipe saddle
Published for 4-inch and larger process pipe.
Standard published maximum
Applies to listed 8-bolt standard configurations.
Legacy saddle support
Specified other-manufacturer and listed WIKA/Ashcroft choices.
| Code | Pipe application | Bolt pattern | Published maximum | Status |
|---|---|---|---|---|
| 3 | 3-in. pipe | 8-bolt | 1,500 psi max | Standard |
| 4 | 4-in. and larger pipe | 8-bolt | 1,500 psi max | Standard |
| 5 | 3-in. pipe | 6-bolt | 1,000 psi max | Optional |
| 6 | 4-in. and larger pipe | 6-bolt | 1,000 psi max | Optional |
| Code | Instrument connection | Published note |
|---|---|---|
| W | Weld connection | Not published with Titanium or Zirconium upper |
| 2 | 1/4-in. NPT | Threaded instrument connection |
| 4 | 1/2-in. NPT | Threaded instrument connection |
| Code | Existing saddle family | Selection note |
|---|---|---|
| X | Other-manufacturer replacement upper | Supply existing saddle details and required geometry |
| W | WIKA 910.ZA / 990.15 | Replacement upper only |
| A | Ashcroft 205 / 206 / 207 | Upper for 3-in. or smaller pipe configuration |
| B | Ashcroft 205 / 206 / 207 | Upper for 4-in. or larger pipe configuration |
| Configuration | Overall width | Lower/body width | Overall height | Body height | Lower height |
|---|---|---|---|---|---|
| 3-in. pipe HSD | 3.50 in. | 3.02 in. | 3.26 in. | 2.50 in. | 0.73 in. |
| 4-in. and larger HSD | 3.50 in. | 3.00 in. | 3.03 in. | 2.27 in. | 0.78 in. |
Dimensions are nominal values reproduced from the published HSD drawing. Confirm the current certified drawing, pipe fit, opening geometry, bolt pattern, and installation clearances before fabrication or welding.
Build the complete saddle assembly
HSD materials, gasket, bolting, installation, and replacement-upper selection
Choose the seal as an integrated pressure boundary: pipe interface, saddle lower, upper, diaphragm, gasket, bolting, instrument connection, fill system, welding procedure, and documentation.
Published wetted materials
Match the lower and diaphragm to the process
- 316 / 316L stainless steel
- 304 / 304L stainless steel
- Tantalum diaphragm
- Alloy 20
- Alloy C-276
- Alloy B-2
- Monel 400
- Alloy 600
- Alloy 2205
- Nickel 200 / 201
- Titanium
- Zirconium 702
- Carbon-steel lower
Material availability depends on the selected component and construction. Titanium and Zirconium diaphragms require matching upper materials in the published ordering logic.
Bolting and option codes
Published assembly and documentation choices
- NN
- Bolting not supplied; standard for other-manufacturer replacement upper
- CS
- Grade 8 zinc-plated bolting
- S4
- 304 stainless-steel bolting
- S6
- 316 stainless-steel bolting
- /AU
- Gold-plated diaphragm
- /NS
- PTFE-coated diaphragm for non-stick service only
- /MC
- Material certification with MTRs
Replacement-upper checklist
Do not select by manufacturer name alone
For a replacement upper, document the existing saddle manufacturer and model, pipe-size family, overall diameter, bolt-circle diameter, number and size of bolts, register or pilot diameter, diaphragm diameter, gasket type and dimensions, upper height, instrument connection, materials, pressure and temperature rating, and any required fill or capillary arrangement.
New installation checklist
Define the pipe and weld before ordering
Provide nominal pipe size, actual outside diameter, schedule, wall thickness, material specification, corrosion allowance, operating and design pressure, operating and design temperature, process opening diameter and location, saddle orientation, weld preparation and procedure, inspection requirements, insulation, accessibility, and applicable piping or pressure-boundary code.
Compare on-line process interfaces
Related Haygor diaphragm seals
Compare saddle, flow-through, pulp-and-paper, and flush-flange geometries when line layout, media behavior, maintenance, or process-connection standards call for a different approach.
Frequently asked questions
In-line saddle diaphragm seal FAQ
These answers summarize Haygor's published HSD product data and practical selection considerations. Final selection must be based on the complete pipe, weld, seal, fill system, instrument, and installation requirements.
Ask Haygor a technical questionWhat is an in-line saddle diaphragm seal?
An in-line saddle diaphragm seal is a pressure-isolation assembly installed directly on a process pipe through a contoured saddle connection. The lower saddle is matched to the pipe, the diaphragm is exposed to process pressure through the prepared pipe opening, and a sealed fill fluid transfers pressure to a gauge, transmitter, or switch while isolating the instrument from the process media.
Which pipe sizes are standard for the Haygor HSD?
Haygor publishes one standard HSD configuration for 3-inch pipe and another for 4-inch and larger pipe. Custom configurations may be available for application-specific pipe geometry; confirm pipe outside diameter, schedule, wall thickness, and saddle fit with Haygor.
What pressure ratings are published for the HSD?
The standard 8-bolt configurations are listed with a 1,500 psi maximum rating. Optional 6-bolt configurations are listed with a 1,000 psi maximum rating. The allowable pressure and temperature of an installed assembly also depend on pipe design, weld procedure, materials, gasket, bolting, instrument system, and applicable plant or piping-code requirements.
How is an HSD saddle seal different from a flow-through in-line seal?
The HSD is mounted to the wall of an existing process pipe through a welded saddle connection. A flow-through in-line seal includes a separate flow body installed as part of the piping run. The preferred design depends on pipe layout, available shutdown and welding work, line size, pressure, media behavior, maintenance strategy, and installation standards.
Can Haygor supply a replacement upper for an existing saddle?
Yes. Haygor publishes replacement-upper selections for specified other-manufacturer saddles, including listed WIKA and Ashcroft designs. The existing saddle geometry, bolt pattern, diaphragm diameter, gasket interface, instrument connection, materials, and pressure rating must be verified before ordering.
Which instrument connections are available?
The published HSD instrument connections are a weld connection, 1/4-inch NPT, and 1/2-inch NPT. The weld connection is not listed with Titanium or Zirconium upper construction.
Which diaphragm and lower materials are published?
Published lower selections include no lower supplied, 316/316L stainless steel, 304/304L stainless steel, Alloy 20, Alloy C-276, Alloy B-2, Monel 400, Alloy 600, Alloy 2205, Nickel 200, Titanium, Zirconium 702, and carbon steel. Published diaphragm choices include stainless steels, Tantalum, Alloy 20, Alloy C-276, Alloy B-2, Monel 400, Alloy 600, Alloy 2205, Nickel 200/201, Titanium, and Zirconium 702.
What information should I provide for an HSD quote?
Provide pipe nominal size, actual outside diameter, schedule and wall thickness, pipe material, operating and design pressure, process and ambient temperature, media and concentration, required bolt pattern, new complete seal or replacement upper, existing saddle manufacturer and dimensions when applicable, instrument model and connection, diaphragm and lower materials, gasket and bolting preferences, fill-fluid or capillary requirements, welding and code requirements, and documentation needs.
Move from pipe data to an HSD configuration
Configure an in-line saddle seal or send the complete pipe application to Haygor.
Use the configurator for a guided starting point, or request a quote for pipe fit, bolt pattern, pressure rating, materials, gasket, bolting, replacement geometry, fill system, instrument, and documentation review.