— A Practical Guide for Engineers, EPCs, and Project Teams
During the detailed design phase of an EPC project, the process team develops Piping and Instrumentation Diagrams (P&IDs) that reflect the piping, engineering, and instrumentation configuration of the process systems. These drawings represent the engineering requirements and components necessary to achieve a safe and compliant process. However, a symbol on a P&ID does not show how the instrument will be physically installed in the field. That is where the Instrument Hook‑Up Drawing comes into play.
The instrument hook‑up drawing is a critical deliverable that bridges the gap between design and construction. It ensures that field technicians install transmitters, manifolds, stands, and cables correctly. Without a clear hook‑up drawing, projects face installation errors, safety hazards, or maintenance difficulties.
1. What Is an Instrument Hook‑Up Drawing?
An instrument hook‑up drawing is a detailed installation schematic that shows how an instrument is to be installed—from the process tapping point on the pipe to the sensing instrument. It specifies pipe slopes, the position of the instrument relative to the tapping point, connection details between the instrument and the pipe, and the bulk materials required for each installation.
The primary purpose of the hook‑up drawing is to ensure that the instrument is installed in a way that allows it to function correctly and to prevent potential problems that could affect measurement, such as liquid traps in gas impulse lines.
1.1 Hook‑Up Drawing vs P&ID
| Aspect | P&ID | Hook‑Up Drawing |
|---|---|---|
| Purpose | Shows which instruments are connected to the process line | Shows how the installation is physically accomplished |
| Detail level | Schematic, functional | Constructible, executable |
| Audience | Process engineers, designers | Field construction technicians |
1.2 Types of Hook‑Up Drawings
Hook‑up drawings are generally classified into:
| Type | Content | Typical Application |
|---|---|---|
| Process hook‑up | Installation details for process control instruments | Pressure, temperature, flow, level transmitters |
| Pneumatic hook‑up | Tubing connections for pneumatic instruments | Control valves, pneumatic actuators |
| Electrical / wiring hook‑up | Cable entry, gland types, junction boxes, grounding | Instruments requiring power or signals |
| Mounting hook‑up | Physical mounting (stands, wall brackets, plates) | All field‑mounted instruments |
2. Input Documents Required
Before starting any hook‑up drawing, you need the following engineering documents:
| Input | What It Provides |
|---|---|
| P&ID | Tag number, tapping point location, process medium (gas/steam/water/hydrocarbon), instrument type |
| Instrument Index | Instrument model, type (AI/AO/DI/DO), hazardous area, calibration range |
| Instrument Datasheet | Installation requirements, connection sizes, material, max operating temperature/pressure |
| Line Isometric / Piping drawings | Exact tapping point locations, root valve location, impulse line routing |
| Electrical Load List | Power requirements, cable termination details |
| Pipe Material Specification (PMS) | Pipe material grades |
3. Standard Contents of a Hook‑Up Drawing
A complete instrument hook‑up drawing typically contains:
3.1 General Information
Tag Number – unique instrument identifier
Loop Drawing Number – reference to the corresponding loop diagram
Layout and Routing Drawing Number – reference to the layout drawing containing the control loop components
Pipe Isometric Drawing Number – reference to the relevant piping drawing
3.2 Installation Details
Position and elevation of instrument relative to the tapping point
Slope direction of impulse lines – to prevent liquid traps or air pockets
Maximum allowable length of impulse lines
Installation elevation of the instrument
3.3 Interfaces and Materials
Identification of the mechanical (pipe/equipment) to instrument interface
Numbering of all components, fittings, and valves
Material Take‑Off (MTO/BOM) – includes part numbers, quantities, sizes, connection types, materials, installation types
3.4 Standard Specifications
Welding specification
Heat tracing and insulation requirements
Pressure testing and painting requirements
4. Step‑by‑Step Procedure for Preparing a Hook‑Up Drawing
Step 1: Identify the Instrument Type
Refer to the P&ID and Instrument Index to confirm:
Instrument type (pressure, temperature, flow, level, or control valve)
Signal type (pneumatic / electric)
Mounting requirements
Step 2: Select the Appropriate Standard Hook‑Up Drawing
Most engineering companies provide a library of standard hook‑up drawings. Common standard configurations include:
Pressure transmitter (direct mount)
Differential pressure transmitter (with 3‑valve manifold)
Temperature element (with thermowell)
In engineering software such as SPI (SmartPlant Instrumentation), the Hook_Up module can generate instrument hook‑up drawings and installation material lists. Establishing a seed library for SPI hook‑up drawings significantly improves design productivity.
Step 3: Draw the Process Connection
Starting from the process tapping point, draw:
The root valve type and location
Impulse line routing and slope (downward for gas, upward for liquid)
Condensate pot (for steam) or seal pot location
Step 4: Add Manifolds and Accessories
Add at the instrument side:
Instrument manifold (2‑valve / 3‑valve / 5‑valve)
Drain/vent valves
Tube fittings and connectors
Step 5: Define the Mounting Arrangement
Determine based on field conditions:
2‑inch pipe stand mounting
Wall bracket
Mounting plate
Support clamps and U‑bolts
Step 6: Prepare the Material Take‑Off (MTO)
List all bulk materials required for each installation:
Stainless steel fittings, tubing
Isolation valves, plugs, manifolds
Transmitter stands (vertical or horizontal)
Sizes, quantities, materials, connection types
Step 7: Label and Review
Complete before issuing for construction:
All dimensions and elevations
Tag numbers, loop numbers
Formal internal and external review
5. Best Practices for Hook‑Up Design
5.1 Prioritise Maintainability
Technicians must have easy access to manifolds, valves, and transmitters for calibration and maintenance. Provide adequate clearance around instruments.
5.2 Consider Safety
Avoid locating instruments near:
Hot surfaces
Rotating equipment
High‑pressure leak points
Electrical panels
5.3 Maintain Consistency
Follow company standard design guidelines (Shell DEP, ADNOC ES, Saudi Aramco SAES, Total, Chevron, ExxonMobil, etc.). Route electrical and tubing runs separately to prevent interference and improve safety.
5.4 Key Considerations for Mounting Position
When developing the drawing, consider these factors to ensure instrument accuracy, repeatability, and personnel safety:
| Medium Type | Instrument Position Relative to Tap | Reason |
|---|---|---|
| Gas | Instrument above the tapping point | Condensate drains back to the line |
| Liquid | Instrument below the tapping point | Gas bubbles rise back to the line |
| Steam | Instrument below the tapping point with a condensate pot | Creates a water seal to protect the instrument |
5.5 Critical Checks Before Issuing for Construction
Before releasing the drawing, the instrument engineer should confirm:
Is the process medium gas, liquid, or steam?
Is there a risk of corrosive media or high/low temperatures to the instrument or personnel? (Consider diaphragm seals or extended impulse lines for cooling)
Is the instrument or valve in a position that is accessible for operation, maintenance, and calibration?
Does the drawing clearly explain material requirements and supply boundaries?
Are all client hook‑up standards followed (e.g., 1:12 slope requirement)?
6. Common Mistakes to Avoid
| Mistake | Consequence | Correct Practice |
|---|---|---|
| Incorrect impulse line slope | Liquid traps in gas lines or gas pockets in liquid lines → measurement errors | Slope down for gas; up for liquid |
| Inaccessible installation | Difficult calibration and maintenance → increased downtime | Ensure manifolds and instruments are reachable |
| Incomplete MTO | Missing fittings on site → project delay | Provide complete MTO with sizes, quantities, materials |
| Ignoring process risks | Corrosion, heat damage to instrument or personnel | Assess need for diaphragm seals or extended impulse lines |
| Not using standard hook‑up library | Inefficient design, inconsistent drawings | Establish a seed library in SPI |
| Poor coordination with piping and modelling teams | Wrong tapping point location | Coordinate with piping team to confirm tapping points |
7. Conclusion
Instrument hook‑up drawings are indispensable deliverables in EPC instrumentation engineering. They connect the symbols on P&IDs to physical field installations, ensuring that instruments are placed correctly, safely, and maintainably.
Key takeaways for hook‑up drawing preparation:
| Phase | Key Task |
|---|---|
| Preparation | Gather P&IDs, Instrument Index, datasheets, piping drawings |
| Design | Identify instrument type → select standard → draw connections → add accessories → define mounting |
| Material | Prepare a complete MTO with sizes, quantities, materials |
| Review | Confirm medium, safety risks, maintainability, client standards |
| Issue | Formal internal/external review before construction release |
Remember: A clear hook‑up drawing saves hours of field interpretation, prevents installation errors, and ensures that instruments remain maintainable throughout their lifecycle.
Contact Us
For instrumentation engineering support, technical documentation, or project quotations, please contact:
Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
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🌐 Website: http://www.tiankang-global.com/
Anhui Tiankang – Supporting EPC projects with reliable instrumentation solutions.
