To pipe a double mechanical seal from its piping diagram, read the plan number and the gland port assignments off the drawings, mount the reservoir beside the pump with its liquid level about 1 m (3 ft) above the gland, and run short, continuously rising tubing from the gland to the reservoir. Then add the plan’s accumulator, cooler or pressure source, connect the vent, drain and instruments, put metal plugs in every port you do not use, and check the loop before the pump starts.

The steps below assume the double mechanical seal is already mounted in the pump, as covered in how to install a cartridge mechanical seal.

Step 1: Identify the Plan and the Ports on the Drawing

Before you cut any tubing, confirm three things: which plan the system follows, which seal zone the plan connects to, and which gland port each line lands on. Plans 52, 53A, 53B, 53C and 54 all connect to the space between the inboard and outboard seals, so their lines go to the barrier or buffer ports, never to the flush port into the seal chamber or to the quench and drain ports on the atmospheric side.

Take the plan number from the piping diagram and the port positions from the seal’s own layout drawing. On a booklet plan page the small seal sketch is an end view from the drive end, and it shows the preferred orientation of the gland connections.

On the seal drawing, find the letter or number codes for the barrier ports. EagleBurgmann, for example, marks them LBI (liquid barrier in) and LBO (liquid barrier out), with Q for quench and D for drain; other makers number the ports and state in the instructions which number does what.

Turn the gland so the ports sit where the piping needs them before you make up any line. Do not connect the piping until the gland bolts are tightened.

Which Gland Port Is the Inlet and Which Is the Outlet

On a plain thermosyphon or pumping-ring loop, the lower gland port is the inlet and the upper gland port is the outlet. The lower (supply) connection on the reservoir goes to the bottom gland port, marked BI or “Buffer/Barrier In”, and the upper (return) connection on the reservoir goes to the top gland port, marked BO.

Seals whose two barrier ports sit side by side at 12 o’clock break that rule. On Chesterton’s 255 cartridge dual seal, viewed from the lock ring end, clockwise rotation sends cool fluid in through the right port and hot fluid out through the left port to the top of the tank; counter-clockwise rotation reverses the two. Pipe such a seal by the maker’s port assignment and the shaft rotation, not by port height.

Buffer or Barrier: What the Arrangement Changes in the Piping

The arrangement changes what the reservoir’s top connection goes to. A Plan 52 buffer system on an Arrangement 2 seal is unpressurized: the reservoir vent is piped to the flare or vapor recovery system through a restriction orifice, with a check valve on the vent line, and stays open in service.

A Plan 53A barrier system on an Arrangement 3 seal has no flare line. Instead, an external gas source, usually plant nitrogen, connects to the reservoir through a pressure regulator and a check valve, and the reservoir is kept pressurized at all times. The reservoir must still be the highest point in the loop so the loop self-vents.

Step 2: Place the Reservoir

Mount the reservoir upright on a rigid stand beside the pump, not directly above it. Set its normal liquid level at least 1 m (3 ft) above the seal gland plate, and keep the reservoir less than 1.2 m (4 ft) from the pump.

If your reservoir’s manual gives its own height rule, check which points it measures between before you set the stand. Height rules are written for the normal liquid level, the reservoir bottom or the reservoir as a whole, measured from the gland plate, the seal chamber or the pump shaft centerline.

Leave room around the reservoir for operation, maintenance and walkways.

Step 3: Size and Run the Tubing

Keep the loop between the gland and the reservoir as short, wide and smooth as you can, because every bit of friction loss in it cuts both thermosyphon flow and the pumping ring’s output. Do not put any valve in the loop that can restrict flow; if ball or gate valves isolate the seal chamber and reservoir, lock them open for filling and operation.

Tubing Size

Use 3/4 in (18 mm) OD tubing on a loop that circulates by a pumping ring or by thermosyphon, which covers Plans 52, 53A, 53B and 53C. Where an external pump or a pressure difference drives the flow, as on Plan 54, 1/2 in (12 mm) OD tubing is enough. Do not go below 1/2 in on any loop.

Use rigid tubing where you can, and make sure it is clean and free of burrs before it goes on.

Slope, Bends and High Points

Run every line upward from the gland to the reservoir at a minimum slope of 1/2 in per foot (about 42 mm per metre), with no local high point where air can collect.

Bend the tubing in long, smooth radii rather than using fittings. Keep elbows, tees and bends to a minimum. If a vapor trap cannot be avoided, add a vent at it.

Thread Sealant at the Fittings

Seal the threaded fittings with an anaerobic liquid thread sealant instead of PTFE tape, because tape can break apart and the pieces travel into the seal. Apply it sparingly, and choose one that is chemically compatible with the process fluid and suits the operating temperature.

Step 4: Connect the Accumulator, Cooler and Pressure Source

Plan 53A pressurizes the reservoir from an external nitrogen supply, while Plans 53B and 53C hold the pressure in an accumulator off the loop and circulate the barrier fluid through a separate cooler:

  • Plan 53A: The reservoir sits in the circulating loop and usually carries its own cooling coil. The pressure source is an external nitrogen supply connected to the reservoir through a regulator and a check valve.
  • Plan 53B: The barrier fluid circulates through a separate cooler, and a bladder accumulator holds the pressure. The gas is needed only once, to precharge the bladder at commissioning, so a portable bottle will do and nothing stays connected.
  • Plan 53C: A piston accumulator holds the pressure, and a reference line from the seal chamber or gland plate to the accumulator supplies it. On a low-pressure pump the reference line has to come from a higher-pressure point instead, such as the pump discharge or an intermediate stage.

The barrier pressure must stay at least 1.4 bar (20 psi) above seal chamber pressure. Work out your own figure with the seal pot pressure calculation for Plans 53A, 53B and 53C before you set the regulator or the precharge.

Plan 53B and 53C Accumulators

The accumulator on a Plan 53B or Plan 53C system sits on a dead leg off the circulating loop, so it can go wherever access and space suit you.

On a 53B system, fit a shade hood or insulation to an accumulator exposed to large swings in ambient temperature, and bleed the vent in its fluid port so no air stays trapped. Set the bladder precharge from the accumulator’s nameplate chart at the ambient temperature at the time of charging.

On a 53C system, mount the piston accumulator vertically with the piston rod at the top, give it a high-point vent and a refill connection, and route the reference line so it does not block access to the pump.

Seal Cooler and Cooling Water

On a Plan 53B or 53C loop, the cooler sits in series in the circulating loop and follows the same placement rules as a 53A reservoir: as close to the seal as possible, a short distance above the seal centerline, and on short, smooth, wide lines.

Pipe the cooling water counter to the barrier fluid, so the coldest water meets the warmest barrier fluid coming from the seal. On Plan 53A the cooling water goes to the coil connections on the reservoir. Vent both sides of the cooler when you first fill it.

Step 5: Connect the Vent, Drain and Instruments

Put the vent at the highest point of the system and the drain at the lowest. On a pressurized loop, fill from the lowest point of the circulating loop so the air is pushed up to the vents; some installations need several vent valves to clear every high point.

Wire the instruments that come with the reservoir: typically a pressure transmitter, a level transmitter or switch, and a level indicator.

Step 6: Plug the Unused Ports

Every gland port the plan does not use gets a solid metal plug before the seal goes into service.

Under the 4th edition of API 682, seals ship with red plastic plugs that carry a yellow warning tag, plus a bag of metal plugs in the correct material. The red plugs are for shipping only. Replace each one with a metal plug from the bag or with the piping.

Follow the maker’s instructions for which ports to plug. Seal the plugs with the same anaerobic sealant as the fittings, applied sparingly.

Step 7: Check the Piping Before Start-Up

Before the pump starts, check the finished loop against the drawing and confirm each of these:

  • Inlet and outlet: The reservoir’s lower connection goes to the gland inlet and its upper connection to the outlet, or, on a seal with ports at 12 o’clock, the ports match the shaft rotation.
  • Rotation direction: The pump turns the way the seal’s pumping device needs.
  • Loop valves: Every isolation valve in the loop is open and locked.
  • Plugs: No red plastic shipping plug is left in any port.
  • Venting and draining: All trapped air can be vented from the system and the interconnecting piping, and the loop can be drained completely.
  • Cleanliness: The reservoir and lines have been flushed with clean fluid; a carbon steel reservoir fogged with rust inhibitor is flushed unless the inhibitor is compatible with the process.
  • Leaks: The piping is leak-tested to your site procedure.