Plant System Drawings and P&ID Interpretation Lab

Why Drawings Are the Operator's First Tool

Before any valve is touched or any pump is started, nuclear plant technicians and operators must be able to read the drawings that describe the plant. Piping and Instrumentation Diagrams (P&IDs), one-line electrical diagrams, and system flow diagrams are the authoritative record of how components are connected and how energy flows through a system. Regulatory and utility procedures require that every system lineup be verified against approved drawings before an evolution begins. Misreading a drawing can send flow the wrong way, leave a critical valve closed, or expose a worker to an unisolated energy source.

P&ID Fundamentals

A P&ID is a detailed schematic showing all mechanical piping, instrumentation, and control elements for a specific system. P&IDs follow the ISA 5.1 standard for instrument symbols and identification. Key symbol categories you must recognize:

• Hand valves (manual valves): Drawn as a bowtie or gate symbol with no actuator shown. Normally labeled with a valve number and position requirement (normally open or normally closed).
• Motor-operated valves (MOVs): Show the basic valve symbol plus a circle or rectangle indicating an electric motor actuator. MOVs appear frequently on residual heat removal (RHR) and emergency core cooling systems.
• Check valves: A standard valve body with an arrow or flap symbol indicating one-way flow. Confirm flow direction against the pipe arrow on the drawing.
• Control valves (CVs): Feature a diaphragm or actuator symbol and are typically connected to an instrument signal line. They modulate rather than simply open or close.
• Instrumentation taps: Small branch lines ending in instrument bubbles. The bubble contains a letter code — for example, FT for flow transmitter, PT for pressure transmitter, TE for temperature element. Each bubble is tagged with a loop number.
• Pump suctions and discharges: Identified by the pump symbol (circle with an internal triangle or blade symbol) and labeled with the suction and discharge line numbers.

Always use the plant's official ISA symbol legend when reading a P&ID — never guess a symbol from memory alone.

One-Line Electrical Diagrams

A one-line (single-line) diagram represents the electrical power distribution system using a single line for each three-phase or single-phase circuit. Key elements include bus bars, circuit breakers, transformers, and motor loads. For nuclear plant operators, one-line diagrams are used to confirm which bus supplies power to safety-related equipment such as MOVs and pump motors. Before performing lockout/tagout, the operator must identify all electrical energy sources on both the P&ID and the one-line diagram.

System Lineup Verification

A system lineup is an ordered list of required component positions (open, closed, energized, de-energized) that must exist before a system is started or placed in service. For an RHR system startup, a typical checklist specifies that suction valves to the reactor coolant system are in a required position, heat exchanger bypass valves are closed, and pump discharge valves are confirmed open to the correct flow path. The technician traces each checklist item on the P&ID, confirms the drawn normal position, and documents any discrepancy — a valve shown normally closed that the checklist requires open, for instance — before proceeding. Never assume a valve position; verify on the drawing and then verify physically.

Lockout/Tagout (LOTO) Applied to P&ID-Identified Isolation Points

Lockout/tagout is governed by OSHA standard 29 CFR 1910.147, the Control of Hazardous Energy standard. It applies whenever maintenance, servicing, or unjamming of equipment could expose a worker to unexpected energization or release of stored energy (electrical, hydraulic, pneumatic, thermal, gravitational, or chemical).

The LOTO sequence for an isolated pump identified on a P&ID:

1. Notify affected employees that a lockout is to be performed.

2. Identify all energy sources using the P&ID and one-line diagram — electrical supply breakers, upstream and downstream isolation valves, any vent or drain paths needed to release stored pressure.

3. Shut down the equipment using normal stopping procedures.

4. Isolate each identified energy source: open the supply breaker, close isolation valves.

5. Apply lockout devices: Place a lock and a DANGER tag on each isolation point. When more than one technician works on the same equipment, use a hasp so each worker applies a personal lock.

6. Release or restrain stored energy: Vent pressurized lines, drain trapped fluid, block against gravity. Wait for thermal energy to dissipate to a safe level.

7. Verify zero energy state (ZES): Attempt to start the pump using the normal control — it must not start. Verify pressure gauges read zero. Use a calibrated voltage tester on the motor terminals.

8. Complete the LOTO log/permit with the time, isolation points, lock numbers, and names of all workers.

Any energy isolation point missed is a life-safety failure. For that reason, the lab check for this lesson treats any missed isolation point as an automatic fail.

Safety Reminders

• P&IDs are controlled documents. Always confirm you are using the current revision before starting a lineup or LOTO.
• PPE during LOTO practice: safety glasses at minimum; arc-flash PPE when working near live electrical panels.
• Never remove another worker's lock. If a lock cannot be identified, follow your facility's abandoned-lock procedure — do not cut it without authorization.
• The LOTO permit must be signed and in place before any isolation point is touched.