A General Guide to Piping Philosophy for P&IDs - WittyWriter

A General Guide to Piping Philosophy for P&IDs

1.0 Introduction & Application

This guide outlines the minimum requirements and general philosophies for piping components shown on Piping & Instrumentation Diagrams (P&IDs). It covers valve selection, general piping arrangements for equipment, and special in-line items. These principles are intended for use during basic, front-end, and detail engineering phases.

2.0 Guidelines for Valve Selection

Valve Function

Manual valves are primarily categorized by their function:

Isolation: Used to stop flow. Requires low flow resistance and bi-directional sealing.
- Typical Types: Gate Valve, Ball Valve, Butterfly Valve, Plug Valve.
Flow Throttling (Control): Used to regulate the rate of flow.
- Typical Types: Globe Valve, V-Port Ball Valve, Needle Valve.
Prevention of Flow Reversal:
- Typical Types: Swing Check Valve, Lift Check Valve.

Isolation Service

The most common isolation valve is the gate valve. For line sizes above 150mm, butterfly valves may be considered. Ball valves (full or reduced port) are also common. Full-port ball valves are often used in slurry services or for safety valve isolation.

Throttling Service

Globe valves are the most common choice for manual throttling, especially in bypass lines around control valves. For very fine control in small lines (≤ 25mm), a needle valve is used. In fluids with suspended solids, a V-port ball valve is preferred as a globe valve's tortuous path can choke.

Flow Reversal Prevention

Check valves (non-return valves) permit forward flow and prevent reverse flow. They are generally not suitable for lines with solids, as particles can prevent a proper seal.

Valve Application & Special Requirements

Fire Safe Valves

Soft-seated valves (like ball, plug, or butterfly) used in hazardous areas must be of a "fire safe" design. This design provides a secondary metal seat that activates if the primary soft seat disintegrates in a fire, maintaining a level of isolation.

Bellows Sealed Valves: Specified for highly toxic or hazardous fluids (e.g., hot oil) where any escape to the atmosphere is unacceptable.
Excess Flow Valves: Designed to close automatically if flow exceeds a set rate, such as from a downstream pipe rupture. Common in LPG service.
Flush Bottom Valves: Used on the bottom of vessels to drain slurries, designed to be pad-mounted and prevent solids from accumulating.
Jacketed Valves: Required for services where the fluid may solidify at ambient temperatures (e.g., molten sulphur).
Cryogenic Valves: Use extended bonnets to keep the valve's packing and operator at a safe temperature.

3.0 General Piping Philosophy

Isolation Valves

Battery Limits: A block valve with a blind (e.g., spectacle blind) shall be provided at all battery limits for isolation.
Steam Lines: Main isolation valves in steam lines must be provided with a smaller "warm-up" bypass valve (typically a globe valve) to allow for gradual heating and pressurization.

Table 1: Warm-up Bypass Valve Sizing
Block Valve Size (mm) Bypass Valve Size (mm)

100 - 200 20

250 - 300 25

350 - 500 40

600 and above 50
Equipment: Block valves are needed on nozzles for operational control or if the vessel contains toxic materials. All storage tank connections below the liquid level must have a block valve.
Pumps: Block valves are required on all suction and discharge piping.
Spared Equipment: Block valves are required to permit the removal of one unit (e.g., a pump) while the spare unit is in operation.
Fired Heaters: The main fuel isolation valve must be located at a safe distance (e.g., 15m) for emergency access.

Table 1: Warm-up Bypass Valve Sizing
Block Valve Size (mm)	Bypass Valve Size (mm)
100 - 200	20
250 - 300	25
350 - 500	40
600 and above	50

Check Valves

Check valves are required on the individual discharge lines of centrifugal pumps, rotary pumps, and centrifugal compressors.
A check valve in a high-temperature (e.g., > 230°C) or congealing service should be provided with a bypass and drain.
Utility connections (steam, water, purge) to process equipment must have a check valve to prevent backflow. A drain valve should be installed between the block valve and check valve.
If the pressure differential between a utility and the process is high (e.g., > 7.5 kg/cm²), two check valves in series (and of different types) should be used.

Control Valve Assemblies

Control valves must have upstream and downstream block valves and a bypass to allow for maintenance. The bypass valve should be a globe valve (up to 200mm) for manual flow control.
Control valves larger than 200mm may not require a block and bypass, but this is a case-by-case decision. If no bypass is provided, a handwheel for manual operation is required.

ESD Valves: Control valves that are tripped open or closed by an Emergency Shutdown (ESD) system must not be provided with isolation valves, as this would defeat their safety function.

All control valve assemblies must have drains (e.DE., 20mm with blind flange) to safely depressurize and drain the section for maintenance.

Table 2: Typical Control Valve Assembly Sizing (mm)
Line Size	Block/Bypass	15	20	25	40	50	80	100	150	200	250	300	350	400
25	Block Bypass	25 25	25 25	25 25
40	Block Bypass	40 40	40 40	40 40	40 40
50	Block Bypass		50 50	50 50	50 50	50 50
80	Block Bypass			50 50	50 50	80 80	80 80
100	Block Bypass				80 80	80 80	100 80	100 100
150	Block Bypass					80 80	100 100	150 100	150 150
200	Block Bypass						150 150	150 150	200 150	200 200
250	Block Bypass							200 200	200 200	250 200	250 250
300	Block Bypass								250 250	250 250	300 250	300 300
350	Block Bypass									300 250	300 250	350 300	350 350
400	Block Bypass										300 250	350 300	400 350	400 400

Pressure Relief Valves (PSVs)

Critical Safety: Isolation of PSVs

Isolation valves on PSVs are a major safety risk. If provided for maintenance, they must be locked open ("LO") during normal operation. PSVs in some services (like boiler steam) may not be permitted to have isolation valves at all.

If a spare PSV is provided, mechanical interlocks are often used to ensure one of the two PSVs is always in service.
Isolation valves must be full-port (full-bore) to prevent flow restriction.
If a rupture disc is installed upstream of a PSV (to protect it), a pressure indicator or switch must be installed in the space between them to detect leaks.

P&ID Notes for PSVs

Inlet Line: Must be sloped back towards the protected equipment. Must be marked "NO POCKETS".
Discharge Line: Must be sloped continuously towards the disposal system (e.g., flare knockout pot). Must be marked "NO POCKETS".
Atmospheric Discharge: Must have a small (e.g., 6mm) weep hole at its low point for drainage. Discharge must be to a "safe location" (e.g., 3m above the nearest platform).

Vents and Drains

All equipment and piping must be provided with valved connections for venting (at high points) and draining (at low points) for startup, shutdown, and maintenance.

Drains in hazardous or toxic service must be routed to a closed drainage system.
Drains in flashing liquid service (e.g., LPG) should be provided with two valves (one block, one globe) spaced apart to prevent icing.

Table 3: Typical Vessel Vent & Drain Sizing
Vessel Volume	Vent Nozzle (mm)	Drain Nozzle (mm)
Less than 6 m³	40	40
6 - 15 m³	50	50
More than 15 m³	50	80

Table 4: Typical Piping Vent & Drain Sizing
Line Size (mm)	Vent/Drain Size (mm)
25 or smaller	Line Size
40 to 300	25
350 and larger	50

Blinds (Positive Isolation)

Permanent blinds (spectacle blinds or spade/spacer sets) are used where positive isolation is required for safety.

At all battery limit connections.
On utility or flushing connections that are not in normal use.
At startup and emergency operation connections.
On spared equipment in hazardous or toxic service.
Spade and spacer sets (instead of spectacle blinds) are used in jacketed, high-temperature, or cryogenic systems to prevent heat transfer.

4.0 Special Piping Items

Steam Traps

Steam traps are required for removing condensate from steam systems (e.g., at exchanger outlets, ejector inlets, and low points in headers). A trap assembly discharging to a condensate system should include upstream/downstream block valves and a bypass for maintenance.

Stripping Steam: For direct steam injection into columns, a steam/condensate separator (boot leg) is required just before the column to prevent slugs of water from entering, which could cause sudden expansion and internal damage.

Bellows (Expansion Joints)

Bellows are used to provide flexibility and protect equipment from piping stresses. They are required at all hard-piping connections to:

Glass-lined vessels and graphite equipment.
Vessels installed on load cells (for weighing).
Vibrating equipment like centrifuges.

Strainers

Strainers are used to protect downstream equipment from particulate matter.

Y-Type: Typically 40mm and below.
T-Type: Typically 50mm and above.
Permanent Strainers: Required at the suction of all pumps, inlet to steam turbines, steam ejectors, steam traps, and burner fuel lines.
Temporary (Conical) Strainers: Used during startup in the suction of process compressors.

Sampling Systems

Low Temp (< 55°C): A simple valve arrangement is used.
High Temp (> 55°C): A sample cooler is required, with the hot process fluid in the coil and cooling water on the shell.
Volatile/Toxic: A closed-loop "sample bomb" assembly is required to capture a sample without venting to the atmosphere.

Locked Valves

Locked Open (LO) / Locked Close (LC)

Valves that must be maintained in a specific position for safety must be identified on the P&ID as "LO" (Locked Open) or "LC" (Locked Close). This ensures their position cannot be changed inadvertently.

Example (LO): Isolation valves for pressure relief devices; pump minimum bypass lines.
Example (LC): Valves on emergency systems not in normal operation.

5.0 Special Piping Requirements

Two-Phase Lines

Lines with two-phase (liquid and gas) flow, or lines subject to water hammer (like boiler blowdown), must be identified on the P&ID. They require special engineering and support to handle the high-energy and vibration.

Tracing & Jacketing

Used to maintain the temperature of a fluid in a pipe.

Tracing (Steam or Electric): Used when accurate temperature control is not critical. Common for "winterization" (preventing freezing) or maintaining viscosity (e.g., furnace oil).
Jacketing: A "pipe-within-a-pipe" design used for fluids that require stringent temperature control (e.g., liquid sulphur, polymers). All valves and fittings in a jacketed line must also be jacketed.

Congealing Services

Lines handling materials that congeal (solidify) must be traced. In some cases, only stagnant areas (like control valve bypasses, instrument lines, drains) are traced. This requires strict operating procedures to ensure lines are drained or flushed before the fluid can cool and solidify.