Last updated: 5/8/2026
Understanding mechanical seal gland symbols is essential for ensuring your pump and seal systems operate reliably and stays within environmental compliance. Whether you are installing a new unit or troubleshooting a leak, correctly identifying connection labels like Flush (F), Quench (Q), and Barrier (BI/BO) is the first step toward a successful setup. This complete guide to seal gland connection labels decodes common industry shorthand and aligns them with standard API flush plans. By mastering these port locations and their specific maintenance requirements, reliability engineers and maintenance technicians can prevent premature seal failure, optimize cooling, and manage fugitive emissions effectively.
Mechanical Seal Gland Port Connections Decoded
F – Flush
- Purpose: To provide cooling to the seal faces by flowing fluid (process fluid from the pump or an external source)
- Connection Type: Process fluid
- Location: Typically at the 12 o’clock position (0 degrees) but could be located anywhere on the seal depending on pump frame or other obstructions
- Where Used: Plan 11, 12, 13, 21, 22, 31, 32, 41
- Maintenance Note: All unused flush ports should be plugged
FI & FO – Flush In & Flush Out
- Purpose: To circulate fluid in and out of the seal to cool the faces
- Connection Type: Process fluid
- Location: Flush In (FI) – 6 o’clock position (180 degrees); Flush Out (FO) – 12 o’clock position (0 degrees)
- Where Used: Plan 14 & 23
- Maintenance Note: In horizontal pump applications, it’s best practice to flush in through the bottom of the seal gland (180 degrees) and out through the top (0 degrees) to ensure the seal remains liquid filled and to promote continuous venting of accumulated vapors. If the seal gland has both an FI & FO port, both should be piped or tubed to an applicable flush plan (14 or 23).
Q & D – Quench & Drain
- Purpose: To inject (Q) and drain (D) an external fluid (typically steam, nitrogen and sometimes water) on the atmospheric side of the seal to prevent buildup or crystallization of the process fluid (such as coking in hydrocarbon applications) on the atmospheric side of the seal faces. Additionally, to provide heating or cooling on the atmospheric side of the seal to prevent overheating or to prevent icing.
- Connection Type: Atmospheric side of the seal
- Location: Quench (Q) – typically at the 3 o’clock position (90 degrees); Drain (D) – typically at the 6 o’clock position (180 degrees) as this is usually the lowest wetted point of the atmospheric side of the seal
- Where Used: Plan 51 & 62
- Maintenance Note: These ports can be left open during normal operation, if acceptable by the operating facility. Some operators prefer to install piping or tubing on the drain (D) port and route it to a safe location (such as a sump) in the event of a seal failure which will convey the leaked process fluid to a safe location.
H & C – Heating & Cooling
- Purpose: To provide heating or cooling to the seal by circulating a heat transfer fluid (typically plant utility water or steam) through a jacket on the seal gland.
- Connection Type: Utility fluid (water, steam, etc.)
- Location: Anywhere on the seal gland or heating/cooling jacket
- Where Used: Heating – high temperature sealing applications where the process fluid is subject to crystallizing or setting up (increasing viscosity) at lower or ambient temperatures. Can also be utilized in such applications during the startup procedure.
- Maintenance Note: A cooling jacket is typically not recommended in high temperature applications due to the potential for plugging or fouling which may reduce efficacy and result in seal failure.
PIT – Pressure Indicating Transmitter or Pressure Sensing Port
- Purpose: To read and transmit the pressure in the seal
- Connection Type: Process fluid or atmospheric side of the seal (refer to the seal drawing)
- Location: Typically at the 3 o’clock position (90 degrees)
- Where Used: Typically Plan 66A & 66B, but could be used in any flush plan to read the pressure either within the seal chamber or seal
- Maintenance Note: Plug if unused
BI or LBI & BO or LBO – Barrier/Buffer In or Liquid Barrier/Buffer In & Barrier/Buffer Out or Liquid Barrier/Buffer Out
- Purpose: To inject and circulate a liquid buffer (unpressurized) or liquid barrier (pressurized) fluid throughout the seal and seal support system
- Connection Type: Process fluid (buffer, if Plan 52 & 55) or Barrier Fluid (barrier, if Plan 53A, 53B, 53C, or 54)
- Location: BI or LBI – 6 o’clock position (180 degrees); BO or LBO – 12 o’clock position (0 degrees)
- Where Used: Unpressurized liquid buffer fluid systems – Plan 52, 55; Pressurized liquid barrier fluid systems – Plan 53A, 53B, 53C, 54
- Maintenance Note: Both the BI or LBI and BO or LBO ports should be piped to the respective flush plan that utilizes a seal pot (e.g. 52, 53A, 53B, 53C, 54, 55)
GBI & GBO – Gas Barrier/Buffer In & Gas Barrier Out
- Purpose: To inject a gaseous buffer (unpressurized) or barrier (pressurized)
- Connection Type: Process fluid (buffer, if Plan 72) or Barrier Fluid (barrier, if Plan 74)
- Location: Gas Buffer In (GBI) – typically at the 3 o’clock position (90 degrees); Gas Barrier In (GBI) – typically at the 12 o’clock position (0 degrees); Gas Barrier Out (GBO) – typically at the 6 o’clock position (180 degrees)
- Where Used: Unpressurized gas buffer systems – Plan 72; Pressurized gas barrier systems – Plan 74
- Maintenance Note: Gas Barrier Out (GBO) should be normally closed and is only used to depressurize the seal during the equipment shutdown procedure.
CSV & CSD – Containment Seal Vent & Containment Seal Drain
- Purpose: To inject a gas buffer fluid (typically a dry, clean inert gas, such as nitrogen) and vent it to the facility’s vapor recovery system. This will collect and convey fugitive emissions (vapors) coming from the inner seal to the vapor recovery system and prevent leakage to atmosphere.
- Connection Type: Process fluid
- Location: CSV – typically at the 12 o’clock position (0 degrees) to facilitate continuous venting of vapors; CSD – typically at the 6 o’clock position (180 degrees) to drain leakage from the primary seal
- Where Used: Plan 72, 75, and 76
- Maintenance Note: CSV should be plugged with Plan 75 (additionally, if Plan 75 is used, the GBI should be plugged if Plan 72 is not used); CSD should be plugged with Plan 76
V – Vent or Process Vent
- Purpose: To vent vapors in the seal chamber or portions of a dual cartridge seals, specifically dual pressurized gas seals that use Plan 74
- Connection Type: Process
- Location: Typically at the 12 o’clock positions (0 degrees) to facilitate venting of accumulated vapors
- Where Used: Typically Plan 74, but can be used in most applications
- Maintenance Note: Plug if not used. Can additionally be used to read seal chamber pressure.
Ready to stop guessing and start installing with confidence? We’ve compiled every major gland port label and symbol into a single, easy-to-read reference sheet. Download your free copy below. Keep it in your toolbox or on your shop wall to ensure every connection is correct, every time.
Still have questions about a specific seal configuration or need help choosing the right piping plan for your application? Our team of sealing experts is here to help you maximize your equipment’s uptime. Contact us with questions.