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This Fluid Sealing Association Knowledge Series training presentation introduces API Piping Plan 11. A description is provided on:
▪ What is an API Plan 11?
▪ How an API Plan 11 Works
▪ What does an API Plan 11 do?
▪ What an API Plan 11 cannot do
▪ Optional Features for an API Plan 11
▪ Cost to Operate an API Plan 11
▪ How to Size an API Plan 11
▪ How to Install an API Plan 11
▪ General API Plan 11 Commissioning Guidelines
▪ How to Operate an API Plan 11
▪ General Troubleshooting of an API Plan 11
▪ Alternatives to an API Plan 11
What Are Piping Plans?
▪ Piping plans collectively are different piping arrangements of fluid used to improve the conditions the mechanical seal operates in with the objective of improving the mechanical seal’s life.
▪ The American Petroleum Institute adopted numbers and created definitions for each piping plan configuration, thereby allowing a common language across the industry to simply describe a particular configuration.
▪ The American Petroleum Institute standard API-682 is where the definition of each piping plan can be found and where they may periodically be updated.
What is an API Plan 11?
API Plan 11 is the most common flush plan in use across the industry. This flush plan simply taps into a pressure source higher than the seal chamber pressure, typically the pump discharge on single stage pumps, or an intermediate stage in a multi-stage pump, and pipes this fluid to the seal chamber to provide cooling and lubrication to the seal faces.
Flow controlling orifice
Mechanical seal or seal chamber “Flush” connection Pump discharge flange
How an API Plan 11 Works
An API Plan 11 works by creating flow using differential pressure between the pump discharge pressure, and the pressure downstream of the seal chamber.
The direction of flow is from the high-pressure source, the pump discharge, to the low-pressure point, downstream of the seal chamber. This fluid flow enters the seal
Flush fluid entry to seal chamber chamber through the flush connection, located in either the mechanical seal gland plate or seal chamber wall.
As the fluid source is the pump discharge, the fluid flowing through the interconnecting piping and seal chamber is the same fluid that is being pumped.
Flush fluid exit from the seal chamber
How an API Plan 11 Works
The flowrate of fluid through the seal chamber can be controlled by adding flow control features. These features include:
Flow Controlling Orifice
Pipe/Tube diameter selection
Flush hole diameter
Seal chamber throat bushing clearance
What does an API Plan 11 do?
The flow of fluid through the seal chamber created by an API Plan 11 can be used to:
▪ Remove heat generated by the mechanical seal. Heat removal can be made more effective when used with a distributed flush to the mechanical seal faces.
▪ Increase seal chamber pressure when used in conjunction with a seal chamber throat bushing to achieve an increased pressure margin above the pumped fluid's vapor pressure.
▪ Prevent the accumulation of debris in the seal chamber.
▪ Allow trapped gases in the seal chamber to vent to the pump discharge before startup.
What an API Plan 11 cannot do
An API Plan 11 cannot:
▪ Isolate the seal chamber from process fluid.
▪ Remove debris from the seal chamber if flow is low.
▪ Cool the process fluid flowing to the seal chamber.
▪ Increase seal chamber pressure without seal chamber throat bushing.
▪ Lower emissions to the atmosphere.
What an API Plan 11 cannot do
The main purpose of an API Plan 11 is to create flow through the seal chamber.
Sufficient flow cannot be created reliably when:
▪ Fluids freeze, solidify, or thicken within the external piping.
▪ Fluids contain high solids and abrasives that abrade and wear the flow controlling components.
▪ Fluids contain large particles that can clog the flow controlling components.
▪ Fluids polymerize and can clog the flow controlling components.
▪ Pumps have a low-pressure differential between the pump discharge and seal chamber that is too low to generate sufficient flow.
▪ Valves are installed in the piping to control flush flow
Optional Features for an API Plan 11
Optional flow control types
▪ Flat Plate Orifice: Narrow plate with a small flow-through hole that restricts flow within the API Plan 11. These are commonly constructed from piping components and can be assembled either seal welded or threaded. Often used in hazardous fluid pumping applications.
Optional Features for an API Plan 11
Optional flow control types
▪ Compression tube fitting with inbuilt orifice restriction.
Constructed using commercially available tubing and compression tube fittings. Offers great flexibility in installation and assembly.
▪ Threaded pipe union with orifice plate. Constructed from piping components and offers advantages where orientation of the connecting pipes is complex.
Optional Features for an API Plan 11
Optional flow control types
▪ Threaded coupling with inbuilt orifice restriction. Often custom fabricated from hexagonal bar stock to meet the design needs.
▪ Choke tube: Length of smaller bore tube, possibly coiled, utilizing flow resistance to reduce the flow through the API Plan 11. Used to control flow rates when high differential pressures exist.
Optional Features for an API Plan 11
▪ To control flowrate in pumps that have a large differential pressure, multiple orifice plates can be used. When using multiple orifice plates, they should be spaced a minimum of 300 mm (12 inches) apart.
▪ The addition of a floating bushing to the seal chamber throat can tighten the clearances at the seal chamber throat and generate an increased pressure in the seal chamber, advantageous for increasing vapor pressure margin around the mechanical seal faces.
Optional Features for an API Plan 11
▪ In multistage pumps, an intermediate stage can be used as the source of the API plan 11 flush, thereby lowering the differential pressure across the API Plan 11 system.
▪ Pressure gauges can be installed upstream and downstream of the orifice to provide the ability to calculate the flow rate and to measure the pressure being delivered to the seal chamber
▪ A temperature gauge can be added to measure the temperature of the flush fluid being delivered to the seal chamber. When installing, ensure the tip of the temperature gauge or thermowell does not obstruct the flow in the piping.
Eccentric reducers to expand pipe size in the region of the thermowell tip
Optional Features for an API Plan 11
▪ For mechanical seal designs where the flush connection is in the seal gland plate, a distributed flush can be incorporated into the mechanical seal design that distributes the flush cooling from a single point to multiple points. This is achieved by creating an annulus that the flush fluid enters and then exits the annulus at multiple points.
Multi-port flush distributor
Annular flush distributor
Cost to Operate an API Plan 11
There is a cost associated with operating an API Plan 11. Energy has been added into the pumped fluid to raise its pressure to the discharge pressure. Most of this fluid flows out of the discharge piping, but some of the flow enters the API Plan 11 flush piping and is eventually returned to the suction of the pump.
The fluid that has flowed through the API Plan 11 piping needs to have energy added to it again to raise its pressure back to the discharge pressure.
This energy consumption, and hence energy cost and carbon footprint, can be calculated as the motor power multiplied by the ratio of the API Plan 11 flush flow rate versus the total pump flow rate:
Cost to Operate an API Plan 11
Typically, the volume of flush is very small compared to the capacity of the pump and therefore the efficiency effect is very small. ▪ Energy to operate $ $$$$
Carbon footprint
Initial investment
Cost of operation
Refer to the Fluid Sealing Association’s Lifecycle Cost Calculator (LCC) for a more detailed analysis.
How to Size an API Plan 11
There are some general rules of thumb often used to estimate required flush flow rates. The most commonly used target is a flow rate of 0.16 l/min per mm of shaft diameter (1 GPM per inch of shaft diameter).
This rule generally works well in cool aqueous pumped fluids; however, a more precise calculation may be needed, particularly with volatile or hot pumped fluids.
The Fluid Sealing Association recommends working with your mechanical seal vendor whenever a more precise target flush flow rate is required.
The calculation method is based on an energy balance to evaluate performance. To make this evaluation the following calculations are needed:
▪ Mechanical seal heat generation
▪ Target flow rate
▪ Actual flow rate through the API Plan 11 piping system
How to Size an API Plan 11
Seal heat generation is influenced by many factors, including seal design, materials of construction, and balance ratio. The governing formulae are:
Where:
Pface = Seal face contact pressure
∆ P = Differential pressure across the seal faces (Sealed Pressure)
B = Balance ratio
K = Pressure gradient
Pspring = Spring contact pressure
Qheat = Seal heat generation
V = Mean velocity
A = Seal face area
f = Coefficient of friction
How to Size an API Plan 11
A target flush rate is necessary to obtain the optimal performance of a mechanical seal as determined by an energy balance calculation. Heat generated between the seal faces is assumed to be absorbed by the flush through ideal mixing. This raises the temperature of the flush as it passes through the seal chamber.
For best practices, the maximum temperature rise through the seal chamber should be limited to:
▪ 8°C (15 °F) for water and low volatility hydrocarbons
▪ 16°C (30 °F) for lubricating fluids
▪ 3°C (5 °F) for volatile/flashing hydrocarbons
Frequently the target flush rate is relatively low, often less than 4 l/min (1 GPM)
How to Size an API Plan 11
The temperature rise occurring with a heat input and ideal mixing occurring can be calculated by:
Where: Δ�� = Temperature rise (T2 – T1)
Qheat = Seal heat generation
ṁ = Flush fluid mass flow rate
Cp = Fluid specific heat
How to Size an API Plan 11
The actual flow through a Plan 11 piping system can be quite complex. Typically, the known pressures are the pump suction and discharge pressure. To calculate the flow the intermediate pressures must be calculated through an iterative process:
How to Install an API Plan 11
An API Plan 11 is typically constructed using tube or pipe components or a combination of both. When using pipe components, the joints can be threaded or, when the pumped fluid is hazardous, welded joints.
When using piping component, the orifice is formed with a plate having a central hole sandwiched between 2 flanges with gaskets.
When using tubing components, the orifice is formed with a tube fitting union with a reduced diameter through hole.
To prevent clogging, the minimum orifice diameter should be no smaller than 3.2 mm (0.125 inches).
How to Install an API Plan 11
API 682 recommends:
▪ For shaft diameters up to 60 mm (2.5 inch):
▪ Pipe: DN15 (½” NPS) Schedule 80
▪ Tube: 12 mm (0.5 inch) 1.5 mm (0.065 inch) wall thickness
▪ For shaft diameters above 60 mm (2.5 inch):
▪ Pipe: DN20 (¾” NPS) Schedule 80
▪ Tube: 20 mm (0.75 inch) 2.0 mm (0.095 inch) wall thickness
High process pressures may require increased wall thickness.
How to Install an API Plan 11
When the connection to the discharge pressure is in the pump volute, typically found in between bearing pumps, use the connections on the side of the volute to prevent solids from being centrifuged into the Plan 11 flush.
How to Install an API Plan 11
To achieve self venting of the seal chamber, the flush connection should enter the seal chamber at the highest point, and the piping connecting the flush connection to the pump discharge should have a continuously rising slope, minimum 40 mm rise per 1000 mm run (0.5 inch rise per 12 inch run).
For pump designs where self venting cannot be achieved, the addition of a high point vent should be added to enable manual venting of a pump prior to commissioning.
General API Plan 11 Commissioning Guidelines
To commission an API Plan 11:
1) Using the mechanical seal assembly drawing, verify that the flush piping is connected to the correct port in the mechanical seal.
2) Check that the orifice size has been indicated on the externally visible surfaces of the orifice.
3) With the pump casing primed, vented, and pressurized prior to startup, check all the joints on the API Plan 11 for leaks. Correct any if found.
4) If a high point vent is incorporated in the API Plan 11 piping, vent the seal chamber following the equipment operator’s standard procedures.
How to Operate an API Plan 11
Operation of an API Plan 11 is inherently connected to the operation of the pump. As soon as the pump generates a differential pressure at the discharge, flow will automatically occur in the API Plan 11 piping.
Periodically check for leaks at the joints. There are no external adjustments that can be made to an API Plan 11 during operation.
During routine maintenance of the mechanical seal, the flow control devices should be inspected for wear, damage, or clogging. Replace or repair as necessary.
General Troubleshooting of an API Plan 11
As flow continuously moves through the API Plan 11 piping, there should be almost no changes in surface temperature along the piping.
A noticeable difference in temperature indicates there is little to no flow occurring, indicating a blockage.
Plugged orifice –The orifice can become plugged if there are particles in the process stream which are larger than the hole in the orifice, or if there are a high concentration of solids or other debris in the process fluid.
Plugged piping (solidification, freezing) – Fluids pumped at elevated temperatures may solidify when they cool down. This can occur in a spare pump on hot standby, in climates where winter temperatures are low, or wind that can cool the external piping.
General Troubleshooting
of an API Plan 11
Inadequate or excessive flow – An improperly sized orifice can result in excessive or insufficient flow rates for the mechanical seal. For between bearings pumps, the API Plan 11 for both ends of the pump may come from a common location. The piping design and orifice locations need to ensure flow is distributed evenly to each seal.
Excessive differential pressure – In multi-stage pumps, the pressure at the discharge may be very high and cause excessive flow that requires complex flow control devices. Connecting the API Plan 11 to the crossover or an intermediate stage can reduce the differential pressure.
Incorrect connection point – Connecting the API Plan 11 piping to the wrong connection port on the mechanical seal can result in the flush fluid not providing the intended cooling or leakage to atmosphere.
General Troubleshooting of an API Plan 11
Excessive flow can cause:
▪ Cavitation due to the high velocities through the orifice. This can be detected as noise occurring at, or downstream of the orifice.
▪ Erosion of the orifice. This can be detected as increased seal chamber pressure
▪ Erosion or cavitation damage of the seal chamber throat bushing. This can be detected as a decrease in seal chamber pressure.
▪ Erosion of the Mechanical Seal from flow impingement directed to a single point. This can be detected as increased atmospheric leakage.
Alternatives to API Plan 11
Alternative piping plans that are similar:
API Plan 11 Summary
▪ Flow to the mechanical seal is created by the differential pressure between the pump discharge (or intermediate stage in a multi-stage pump) and seal chamber.
▪ Flow is used to:
▪ Remove heat generated by the mechanical seal. Heat removal can be made more effective when used with a distributed flush to the mechanical seal faces
▪ Increase seal chamber pressure when used in conjunction with a seal chamber throat bushing to achieve an increased pressure margin above the pumped fluid’s vapor pressure
▪ Prevent the accumulation of debris in the seal chamber
▪ When connected correctly the API Plan 11 piping can also allow trapped gases in the seal chamber to vent to the pump discharge
