A P&ID tells you that a transmitter exists, but not how to connect it to the pipe. The installation sketch fills that gap, showing every valve, fitting and metre of tubing the site team needs to mount the instrument correctly.
Correct installation decides whether a good transmitter gives good readings. This guide explains what installation typicals contain, how they differ for gas, liquid and steam and how to calculate slope and tubing length.
What Is a Hook Up Drawing?
A hook up drawing is an installation detail that shows how an instrument connects to the process or to its air and signal supply, including the root valve, impulse or tubing route, fittings, manifold, mounting support and a bill of materials. It turns the symbol on a piping and instrumentation diagram into a buildable arrangement.
Automation Forum describes these drawings as the ones that represent installation standards. They show how each instrument sits in the plant and which materials are needed.
What the Drawing Shows
Automation Forum separates process hook ups, connected to the process line, from pneumatic hook ups for air and signal tubing. Both types list items such as valve manifolds, fittings and supports.
Bill of Materials on a Hook Up Drawing
| Item | Typical Specification | Supplied By |
|---|---|---|
| Root valve | Gate or globe, per piping class | Piping |
| Tubing | Seamless SS 316, 12 mm or 1/2 inch | Instrument |
| Compression fittings | Twin ferrule, SS 316 | Instrument |
| Manifold | 3 valve or 5 valve, SS 316 | Instrument |
| Support | 2 inch pipe stand with U bolts | Instrument |
The split of supply between piping and instrument contractors must be clear on the hook up drawing. Ambiguous boundaries are a common cause of missing parts at site.
Impulse Line Slope and Routing
Impulse lines slope continuously so that gas vents back to the process in liquid service and liquid drains back in gas service. A common rule is at least 1 in 12, as explained in impulse line installation slope.
Electrical and Control warns that piping must never form traps where gas bubbles or liquid sumps can collect. Basics are covered in what are pressure sensing lines.
6 Essential Typical Hook Ups
Tap from the side, transmitter below the tapping.
Tap from the top, transmitter above the tapping.
Taps on the horizontal side, lines fall to the transmitter.
Taps on the top half, lines rise to the transmitter.
Condensate pots, lines filled with water, transmitter below.
Wet or dry leg, or remote seals on a vessel.
Electrical and Control explains that liquid lines should come from the lower half of the pipe and fall to the transmitter, while gas lines come from the top and rise. For steam, unlagged lines below the pipe fill with water and protect the diaphragm.
Steam typicals often include condensate pots, and level typicals follow DP transmitter level measurement. Remote seals with capillary tubing appear where fluids are hot, viscous or corrosive.
Slope and Tubing Formula
Tubing length = Length × Number of lines × (1 + Allowance ÷ 100)
Example:
Horizontal run 3 m, slope 1 in 12
Drop = 3 × 1000 ÷ 12 = 250 mm
Two impulse lines, 10% allowance for bends
Tubing = 3 × 2 × 1.1 = 6.6 m
Plan 250 mm fall and 6.6 m of tubing
The allowance covers bends, offsets and cutting waste.
Hook Up Drawing Slope Calculator
Reading and Using the Drawing at Site
The finished installation is proven during the instrument loop checking procedure. Good practice for mounting is summarised in pressure transmitter installation best practices.
Where lines may freeze or the fluid may solidify, the drawing adds tracing details, see heat tracing in piping systems. The VorCone installation guide also warns never to use dissimilar metals in impulse lines, because this can cause corrosion.
- Consistent installation across the plant.
- Accurate material take off.
- Faster site work and inspection.
- Fewer measurement errors from trapped fluid.
- Site routing may differ from the sketch.
- Generic typicals miss special cases.
- Needs updates after field changes.
- BOM errors multiply across many tags.
A hook up drawing works together with the loop diagram, which shows wiring, and the location plan, which shows where each instrument sits. Keep all three documents at the same revision.
Installation Guide PDF
Hookup Drawing Video
Hook Up Drawing FAQ
It is an installation detail that shows how an instrument connects to the process or to its supplies. It includes root valves, tubing, fittings, supports and a bill of materials.
One sheet often serves many tags that share the same arrangement. The tag list printed on the sheet shows exactly where it applies across the plant.
A P&ID shows which instruments exist and how they relate to the process lines. It does not show fittings, slopes, supports or the exact mounting position of each device.
The installation sketch adds all of those physical details for the site team. Fitters and supervisors build the arrangement directly from it, together with the bill of materials.
A slope lets gas vent back to the pipe in liquid service and liquid drain back in gas service. Trapped bubbles or liquid sumps cause false and drifting readings.
A common minimum slope is 1 in 12 along the whole route. Avoid any high or low pockets between the tapping point and the transmitter body.
For gas service the transmitter usually sits above the tapping point on the pipe. Any condensed liquid then drains back into the pipe instead of the sensing lines.
Taps are taken from the top half of the horizontal line, usually within 45 degrees of the top. This arrangement keeps liquid out of the impulse lines during normal operation and after shutdowns.
The transmitter sits below the taps with both impulse lines filled with water. Condensate pots at the tapping level help keep equal liquid legs on both sides.
The water protects the diaphragm from direct contact with live steam. Fill both legs fully before putting the meter into service after any maintenance work or a long plant shutdown.
It lists root valves, tubing, fittings, manifolds and supports with their sizes and materials. Quantities are normally given per tag so project totals can be multiplied easily.
The list is used for purchase and for issuing material to fitters at site. Any error repeats across every tag that uses the same typical, so check it carefully.
The instrument design team prepares them during the detailed engineering phase. Piping and process teams review the tapping details and ratings before issue for construction.
Site teams mark any field changes in red on their working copies. The final updated set is then issued to the client as built for plant records and future maintenance.
Related Articles
- Piping and Instrumentation Diagram
- Impulse Line Installation Slope
- What Is a Valve Manifold
- Pressure Transmitter Installation Best Practices
- Condensate Pot in Steam Applications
External References
- VorCone Installation Guide, Vortek Instruments
- How to Connect a DP Flow Sensor to a DP Transmitter, Electrical and Control
- Piping and Instrumentation Diagram, Wikipedia
What We Learn Today
- A hook up drawing shows how an instrument connects to the process, with root valves, impulse routing, manifolds, supports and a complete bill of materials.
- Liquid lines fall to a transmitter below the tap, gas lines rise to one above, and steam uses water filled legs with condensate pots.
- Keep impulse lines at a slope of at least 1 in 12, avoid pockets, and plan tubing with an allowance for bends and offsets.