A throttle bushing is a close-clearance ring fitted into the atmospheric side of the gland plate, encircling the shaft sleeve just outboard of the seal faces, with its bore held as small as practical so that it restricts what can leave the gland. API 682 treats it as a containment device — part of the seal’s specification, in the same field of the seal code as the containment seal that can take its place.
What a Throttle Bushing Does
A throttle bushing contains leakage after a primary seal failure, channels routine leakage to the gland drain, and makes seal leakage measurable. It is not a sealing element.
Those jobs split by flow regime. At normal leakage rates the bushing restricts little; its shape at the gland exit steers liquid into the drain connection. When leakage becomes excessive, the minimised bore throttles the flow escaping to atmosphere. Against gaseous leakage, a fixed bushing offers almost no containment or channelling at all.
The containment API 682 asks for is a shutdown window: its stated objective is to alarm the operator once leakage exceeds normal levels and then contain effectively for a short period — long enough to shut the equipment down without disrupting the process.
It also does not support the shaft, does not raise seal-chamber pressure and does not cut flush consumption; those claims belong to the throat bushing.
Where the Throttle Bushing Sits
It sits in the gland plate, on the atmospheric side of the seal faces, around the shaft sleeve. The gland drain and any leak-detection connection open into the cavity between the bushing and the faces, so everything passing the seal reaches that cavity first and the bushing second.
The throat bushing is a different part at the opposite end of the seal: it fits in the throat of the seal chamber, between the seal and the impeller, and its job is the seal chamber environment. The two sit side by side in our throat bush vs throttle bush comparison.
Fixed, Floating and Segmented Throttle Bushings
The three variants differ in the clearance they can hold at the sleeve. A fixed bushing is a ring inserted into the gland plate and runs the loosest. A solid floating bushing is a carbon graphite ring located axially by springs but free to move radially and centre itself on the sleeve, and runs tighter. A segmented carbon bushing splits that ring into garter-spring-retained segments with designed contact against the sleeve, and closes tighter still.
API 682 puts numbers on the first two. A fixed throttle bushing’s diametral clearance may not exceed 0.635 mm (0.025 in) for sleeve diameters up to 50 mm (2 in), plus a further 0.127 mm (0.005 in) for each additional 25 mm (1 in) of diameter. A floating carbon bushing is held to 0.18 mm (0.007 in) up to a 50 mm sleeve, 0.225 mm (0.009 in) from 51 to 80 mm, and 0.28 mm (0.011 in) from 81 to 120 mm, measured at pumping temperature. A segmented bushing is built to touch the sleeve and, after a wear-in period, typically runs a diametral gap of about 0.04 mm (0.0015 in), which narrows further under a pressure differential across it.
API 682 makes fixed bushings the default on Arrangement 1 Category 1 and 2 seals, requires the floating designs on Category 3, and its Fourth Edition hazard-based selection logic can call for a floating carbon bushing on an Arrangement 1 seal.
For leak detection, or anywhere the leakage arrives as gas, specify floating or segmented. A solid floating bushing is designed across a temperature range, so its clearance is widest at the low temperatures typical of services whose leakage flashes at ambient — exactly the duty where a segmented design is far stronger.
How a Throttle Bushing Triggers a Seal Leak Alarm
The bushing converts a rise in seal leakage into a pressure signal an instrument can read, and that mechanism is the basis of the Plan 65 and Plan 66 detection schemes.
A Plan 66A puts two close-clearance throttle bushings into the gland behind the seal face. Leakage is restricted by the inner bushing and raises the pressure behind the face, where a pressure transmitter reads it. Leakage that gets past flows into the drain cavity at low pressure and goes on to the Plan 65 system.
A Plan 66B uses one close-clearance bushing in the gland and a plug orifice in the drain line instead. Leakage entering the drain cavity is partially restricted from flowing out, so a high leakage rate pressurises the cavity and the transmitter picks it up. It is less sensitive than the 66A and adapts easily into existing seal glands.
Downstream, Plan 65A detects the leakage rate and Plan 65B detects accumulated leakage in a collection vessel, both using a level transmitter for trending; API 682 treats them as technically equivalent ways of satisfying a Plan 65. All of this sits around a single seal with a containment device, and the full set of arrangements is on our API plans page.
Throttle Bushing Materials
Throttle bushings are made of carbon graphite or a non-sparking metal such as bronze, because the part has to run within a few tenths of a millimetre of a rotating sleeve, survive contact without galling or sparking, and shed nothing back toward the seal.
API 682 ties the fixed-bushing material to the seal category: carbon graphite is required for Category 1, while a non-sparking metal such as bronze is specified for Category 2. Floating and segmented bushings are carbon graphite in both cases.
One carbon bushing design cannot cover the standard’s full temperature range, so a service at either extreme needs its clearance checked at temperature.
Where both properties are wanted at once, the retaining plate can be built in bronze and machined to a fixed-bushing clearance around a floating carbon bushing: the float keeps the tight clearance and the bronze plate keeps the non-sparking surface.
When a Containment Seal Replaces the Throttle Bushing
A containment seal takes over from the throttle bushing as the containment device when bushing-level containment is not enough — most often when the leakage reaches the gland as vapour, where even a floating bushing passes too much.
The containment seal is the outer seal of an Arrangement 2 dual unpressurised seal, running on buffer gas or vaporised process fluid, contacting or non-contacting. Its support plans differ accordingly: Plan 71 for the dry-running containment seal itself, Plan 75 to collect and monitor liquid leakage, Plan 76 for vapour leakage.
Specifying one does not delete the bushing from the assembly. API 682 still requires a fixed non-sparking bushing inboard of the containment seal faces, downstream of the containment chamber vent connection, to isolate those faces from process leakage.
The bar a containment seal has to meet is set out in the standard: at least 25,000 hours at any containment chamber pressure up to the seal leakage pressure switch setting, which is capped at 0.7 bar gauge (10 psi), and at least 8 hours at full seal chamber conditions.
On paper the choice shows up in the containment-device field of the API 682 seal code, which distinguishes a plain gland, a fixed bushing, a floating bushing, a segmented bushing and a containment device. Published letter lists disagree on which letter means a plain gland and which means a floating bushing, so take F for a fixed bushing, C for a containment device, S for a segmented bushing and X for unspecified, and read the remaining two off Annex D in your own copy of the standard before they go onto a purchase order.
Which device a duty needs stays a judgement. The criterion is the hazard or pollution potential of both normal leakage and failure leakage, filtered through corporate, plant and regulatory requirements; the Fourth Edition adds a selection path keyed to the GHS fluid hazard classification.