Read an API seal plan drawing in two layers: the piping, valve and vessel symbols that show where the fluid goes, and the instrument tags that show what is measured along the way. Start at the gland connections, follow each line out to the support equipment and back, and decode every bubble you pass with the instrument letter rules.
A seal plan drawing is a small piping and instrumentation diagram (P&ID) of the seal support system. Most of its symbols are ordinary P&ID symbols.
Which Standards Define the Symbols on a Seal Plan Drawing
API 682 defines the piping plans themselves, including the instrumentation each plan requires and the design of the seal support equipment, while ISA-5.1 defines the instrument symbols and tag letters drawn on them. The current edition of API 682 is the fourth, published in May 2014. The third edition of API 682, issued in 2004, was aligned with ISO 21049, so older drawings may cite ISO 21049 instead.
For the piping and equipment symbols, projects in the chemical and petrochemical industry often follow ISO 10628-2:2012.
Read the drawing’s legend sheet before anything else. Where a symbol in this article and the legend disagree, follow the legend.
How to Read the Connection Codes at the Gland
On an API figure, the gland connections are numbered callouts, and a key below the drawing gives each number its name and letter code. On the API 682 Plan 65 figure, for example, the key reads “2 — Flush (F)”, “3 — Quench (Q), plugged” and “4 — Drain (D)”. The letters are abbreviations in the key, though other drawings print them straight beside the port.
The connection codes you will meet on seal plan drawings are:
- F: flush, the connection that delivers flush fluid to the seal chamber.
- FI / FO: flush inlet and flush outlet, used where a seal has separate flush ports.
- Q: quench, the atmospheric-side connection for steam, water or nitrogen.
- D: drain, the atmospheric-side drain.
- V: vent, at the high point of the seal chamber.
- LBI / LBO: liquid buffer or barrier inlet and outlet, the two ports of a dual seal’s liquid loop.
- GBI / GBO: gas buffer or barrier inlet and outlet, the same pair on a gas-supported dual seal.
- CSV / CSD: containment seal vent and containment seal drain, on a dual seal with a dry containment seal.
How to Read the Instrument Tags
An instrument tag reads left to right: the first letter is the variable measured, the letters after it say what the instrument does with it, and the number is the loop it belongs to. On a seal plan the first letter is almost always P (pressure), PD (differential pressure), L (level), F (flow) or T (temperature). The succeeding letters are usually I (indicate), T (transmit) or S (switch), with H or L for a high or low alarm point.
So PIT is a pressure transmitter with a local indicator, LSL is a low-level switch, and PDIT is a differential pressure transmitter with a local indicator. FV or FCV is the valve in a flow loop: the first letter names the variable the valve controls and the V marks it as a valve.
The edition of the drawing changes which tags you see. API 682’s fourth edition made transmitters with local indicators the default instrument on piping plans, replacing the switches and gauges of earlier editions.
The shape of the bubble and any line through it tell you where the instrument is mounted or which system reads it, such as local, a control room panel or a PLC. The conventions vary between projects, so take them from the legend sheet.
When FI Means Flush Inlet and When It Means Flow Indicator
FI is a flow indicator when the letters sit inside an instrument bubble, and a flush inlet when they sit beside a gland port or in the connection key.
The same collision happens with FO. Beside a gland port it means flush outlet, but written under a valve on a general P&ID, FO means the valve fails open.
How to Read the Line Types
A solid line is process piping, and the instrument signal lines are told apart by the marks along them. ISA-5.1’s line chart draws them as follows:
- Solid line: process piping and the seal plan’s fluid connections.
- Dashed line, or solid line with triple slashes: electrical signal.
- Solid line with double slashes: pneumatic signal.
- Solid line with “L” marks: hydraulic signal.
- Solid line with small circles: mechanical link.
ISA’s chart has no dotted line at all, so on a drawing that uses one, the legend has to define it.
How to Read the Equipment and Valve Symbols
Seal plan drawings use the ordinary P&ID symbols for valves and pipe fittings, plus a handful of vessels and restrictions that are specific to seal support systems.
Valves and Relief Valves
A valve is drawn as a bowtie, two triangles meeting at their points, and the gate valve and globe valve often share that same symbol. A globe valve is sometimes drawn with a filled dot where the triangles meet. When the symbols are identical, the valve type is in the tag or the notes, and sometimes not on the drawing at all.
A relief valve is drawn on an angle body, with the inlet and outlet at right angles, and a spring on the stem. A valve with no actuator on it is a manual valve. A control valve is the valve symbol with an actuator symbol on the stem.
In general P&ID practice, an open bowtie is an open valve and a filled bowtie is a closed one, and the letters FO, FC and FAI beside a valve mean fails open, fails closed and fails as is. Check the legend before you read a filled bowtie as a closed valve.
Orifices, Strainers, and Cyclone Separators
The orifice is drawn as two short parallel bars across the line. On a general P&ID that symbol is a flow-measuring element. On a seal plan it is a restriction that sets the flow, as in the flush line from the pump discharge to the seal on Plan 11.
A strainer is drawn either as vertical bars across the line or as a small box on the line with a chevron inside.
The cyclone separator, used on Plan 31 and Plan 41, is drawn as an inverted cone. Read its connections: the inlet from the pump discharge enters at the side, the clean outlet at the top goes to the seal, and the dirty outlet at the bottom returns to pump suction.
Reservoirs, Coolers, and Accumulators
The Plan 53A reservoir, or seal pot, is drawn as a tall vertical vessel with level instruments on its side and a gas line entering at the top.
The seal cooler is drawn as a circle crossed by a zigzag line, with the coolant connections at two sides and usually a temperature indicator tied to it. You will find it in the loop of Plan 23, Plan 53B and Plan 53C.
The bladder accumulator of Plan 53B is drawn as a closed, rounded capsule standing on a branch off the cooling loop. That branch is a dead leg: the barrier fluid circulates through the cooler, not through the accumulator. The low-level alarm is taken from pressure, so expect a pressure instrument on the accumulator and no level bubble.
The piston accumulator of Plan 53C is drawn as a vertical cylinder with a rod sticking out of its top, again on a dead leg off a loop with a cooler. Look for the extra line from the pump seal chamber to the accumulator.
How to Trace a Seal Plan Drawing From End to End
Trace a seal plan drawing by starting at the port where the fluid leaves the seal and following the line, in the direction of the flow arrows, through every vessel and instrument until it returns to the inlet port. On a Plan 53A drawing that loop runs as follows:
- Leave the seal at LBO: the barrier fluid exits the gland at the liquid barrier outlet, and the line from LBO runs up into the reservoir near its top.
- Pass through the reservoir: the fluid is cooled by the coil inside the vessel.
- Return to LBI: the line leaves the bottom of the reservoir and runs back to the liquid barrier inlet, where an arrowhead points into the gland.
- Read the pressure side: the nitrogen line enters the top of the reservoir through a valve, with the pressure instruments on that gas line.
There is no pump in the loop: the circulation comes from a pumping ring inside the seal cartridge. A third-edition drawing hangs PI, PSL, LI, LSH and LSL bubbles on this loop; on a fourth-edition drawing of Plan 53A, expect transmitters in their place.
Where a line runs off the sheet, it ends in a pointed tag naming the drawing and grid coordinate it continues on, and the direction of the point gives the direction of the flow. Valves are drawn in their normal operating position unless the drawing says otherwise, and a valve shown closed is part of the path only when someone opens it.