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Brand: Flowserve
Flowserve QBU Hot Water Pusher Mechanical Seal
Description
Flowserve QBU hot water pusher seal
The Flowserve QBU is a single pusher mechanical seal developed specifically for hot water duties, such as moderate-pressure boiler feed and circulation pumps. Its distinguishing feature is Lube Groove face technology in the carbon rotating face, which creates a stable fluid condition at the faces and lets the seal run at higher temperatures than a conventional flat-face seal.
The QBU is supplied as a cartridge or as a component assembly. Because the seal is built from the same family as the conventional QB seal and shares interchangeable parts, it is an easy upgrade for plants that already hold QB spares. This page describes the QBU series; the final seal is defined by shaft size, materials and flush plan. Seven Star LLC is an independent supplier in Muscat and handles Flowserve seal enquiries for Oman and the GCC.
QBU technical specifications
| Parameter | Specification |
|---|---|
| Manufacturer / part number | Flowserve / QBU |
| Seal type | Single pusher seal with Lube Groove carbon face; cartridge or component |
| Pressure | Up to 17.6 bar (255 psi) |
| Vapor pressure margin | Minimum 2 bar (30 psi) |
| Temperature, Plan 11 | Up to 170 °C (340 °F) |
| Temperature, Plan 21 | Up to 204 °C (400 °F) |
| Shaft speed | 750 rpm up to 20 m/s (65 fps) |
| Shaft size | 12.7 to 139.7 mm (0.500 to 5.500 inch) |
| Metal components | 316 stainless steel |
| Rotating face | Carbon |
| Stationary face | Silicon carbide |
| Gaskets | Fluoroelastomer, perfluoroelastomer or EPR |
| Springs | Alloy C-276 |
How the QBU seal works
Hot water close to its boiling point is a hard fluid for a seal, because a small rise in face temperature can flash the liquid film and cause face damage. The Lube Groove features cut into the carbon face keep a stable liquid layer between the faces, so the seal tolerates more heat and does not rely on cooling water in the same way as a flat-face seal. The manufacturer requires at least 2 bar of margin between seal chamber pressure and the liquid vapor pressure so that the film stays liquid.
A multiport flush in the gland improves heat removal and keeps the faces at a uniform temperature, which supports long seal life. A pusher design means the secondary seal slides along the shaft or sleeve to follow face wear, and the springs push the carbon face against the silicon carbide stationary face.
Where the QBU fits in the seal range
The QBU is the moderate-pressure choice among Flowserve hot water seals. For high-pressure boiler feed and circulation pumps, injection and pipeline services the manufacturer offers the DHTW and UHTW seals, which handle pressures to 207 bar. The QBU suits the lower-pressure group where a single pusher seal with a simple Plan 11 or Plan 21 flush is enough, and it can be fitted to the pump types common in industrial boiler houses and utility plants.
QBU selection and ordering checklist
- Pump make and model, shaft or sleeve diameter and seal chamber dimensions
- Liquid temperature and seal chamber pressure, to check the 2 bar vapor margin
- Pump speed in rpm and the resulting surface speed
- Flush plan: Plan 11 up to 170 °C or Plan 21 above that to 204 °C
- Gasket material: fluoroelastomer, perfluoroelastomer or EPR, matched to water chemistry
- Cartridge or component seal
- Quantity of seals and spare parts requirement
- Existing seal type, if the QBU replaces a QB or other seal
The datasheet notes that additional sizes and conditions not listed can be requested from the manufacturer.
Installation, commissioning and maintenance
Check the shaft or sleeve surface finish, shaft runout and seal chamber dimensions before installation, and fit the cartridge exactly to the setting instructions. Open the flush lines and vent the chamber before running, because dry running destroys carbon faces quickly.
With a Plan 21 cooler, keep the cooler clean and monitor outlet temperature; if the temperature climbs above design, the face film can flash. During operation watch for leakage steam or visible wear dust, and plan face and elastomer replacement during pump overhauls. Spare seal kits containing faces, gaskets and springs are the most useful items to keep in stock, particularly because parts interchange with the QB family.
Typical QBU applications in Oman and the GCC
Typical uses are boiler feed pumps and boiler circulation pumps in industrial steam systems, power plant auxiliary services and hot-water circuits in desalination and process plants. In the hot climate of Oman, cooling water is often scarce or warm, so a seal that runs without external cooling up to 170 °C is attractive for saving utilities. Water quality and carryover of deposits should be considered when choosing the elastomer.
QBU sample size and dimension table
| Seal size | Shaft A max, mm (inch) | B max, mm | C min, mm | D min, mm |
|---|---|---|---|---|
| 0875 | 12.70 (0.500) | 43.64 | 35.71 | 51.59 |
| 1000 | 15.88 (0.625) | 46.81 | 35.71 | 51.59 |
| 1500 | 25.40 (1.000) | 59.51 | 36.50 | 56.36 |
| 2000 | 38.10 (1.500) | 75.30 | 49.23 | 63.50 |
| 3000 | 63.50 (2.500) | 107.1 | 57.15 | 70.64 |
| 4000 | 88.90 (3.500) | 131.7 | 54.76 | 76.99 |
| 6000 | 139.70 (5.500) | 195.2 | 61.90 | 90.50 |
The datasheet lists sizes from 0875 to 6000 in steps; the table above is a selection. Request the full table for the exact shaft diameter.
Cooling and flush considerations for hot water
Hot water above roughly 80 °C has little lubricating ability and can flash across the seal faces, which causes face wear and noise. For boiler feed, condensate and hot water circulation duty it is common to cool the seal chamber with API Plan 23, where a pumping ring circulates a small volume of water through a cooler in a closed loop, or to use an external flush. Keep the seal chamber temperature well below the boiling point at chamber pressure, use suitable face materials such as silicon carbide against carbon, and select elastomers rated for hot water such as EPDM.
Information to send with a QBU RFQ
- Pump data, shaft diameter and seal size
- Liquid, temperature, pressure and speed
- Flush plan and cooling water availability
- Elastomer preference and cartridge or component form
- Quantity and delivery location
Documents and quotation
A datasheet for the QBU seal is attached on this page. Send the shaft size and operating conditions and we will confirm the seal selection and arrange a quotation.
Seven Star LLC is an independent supplier and does not claim any official status with the manufacturer.
Datasheets & Documents
- Datasheet Flowserve QBU Seal Datasheet Download
Reviews (0)
About brand
Flowserve is the American flow control company for pumps, valves, mechanical seals and valve automation, known through brands such as Durco, Worthington, Byron Jackson, Valtek, Limitorque, Worcester, Logix and Argus. It was formed on 22 July 1997 by the merger of equals of Durco International and BW/IP, whose heritage reaches back to Simpson & Thompson (1790) and Byron Jackson (1872). Flowserve Corporation is headquartered in Irving, Texas, and listed on the New York Stock Exchange.
Seven Star LLC sources and supplies Flowserve pumps, seals, control valves, ball valves, actuators and positioners for process plants in Oman, working from the nameplate data and the manufacturer's documentation.
Flowserve company profile
| Formed | 22 July 1997, merger of Durco International and BW/IP |
|---|---|
| Heritage companies | Simpson & Thompson, later Worthington Simpson (1790); Byron Jackson (1872); Duriron foundry, later Durco (1912) |
| Headquarters | Irving, Texas, USA |
| Ownership | Flowserve Corporation, public company (NYSE: FLS) |
| Product families | API and ANSI process pumps, Durco and Worcester ball and plug valves, Durco TX3 triple-offset and Valtek butterfly and globe control valves, Limitorque electric actuators, Logix positioners, mechanical seals |
Milestones
- 1790: Thomas Simpson founds Simpson & Thompson, a predecessor of Worthington Simpson pumps.
- 1872: Byron Jackson is founded, a root of BW/IP.
- 1912: The Duriron foundry, later Durco, is launched.
- 1997: Durco International and BW/IP merge on 22 July to form Flowserve, listed on the NYSE as FLS.
Flowserve product families listed here
Flowserve pumps on this page are the Durco Mark 3 ANSI process pump and the ECPJ vertical sump pump built to API 610 type VS4. Sealing products are the ISC2 cartridge mechanical seal to API 682 and DuraClear barrier fluid for dual seal systems. Flowserve valves include the Valtek Mark One globe control valve, Argus floating and trunnion-mounted ball valves (EK71, FK79, FK76M) and Durco Microfinish ball valves. Automation is covered by Limitorque MX and L120 multi-turn electric actuators, the Rotadisk RD and RDF pneumatic actuator and the Logix 3800 digital positioner.
Where Flowserve equipment is used
Mark 3 pumps handle acids, caustic, solvents and utilities in chemical and petrochemical units. VS4 sump pumps drain pits and collection tanks. Valtek control valves and Argus ball valves are found in refining, gas processing, power and desalination, and Limitorque actuators operate gate and globe valves on pipelines, tank farms and water networks across the GCC.
Flowserve products we list
Ordering Flowserve parts: what we need
The serial number on the nameplate is the key to Flowserve records for a pump, valve or actuator. Please add the following by product type:
- Pumps: size and model, impeller diameter, materials, seal arrangement, and the part name and item number from the sectional drawing
- Mechanical seals: seal type and size, drawing or part number, materials code, pumped fluid and flush plan
- Valves: size, pressure class, body and trim materials, end connections, and the actuator and positioner model
- Limitorque actuators: unit size, order and serial number, supply voltage, wiring diagram number
Facts checked against Flowserve's SEC filings and published company histories, October 2026.
Browse Flowserve products by category: Control Valves, Pumps, Valve Actuator, Valve Positioner.
FAQ
Questions about the Flowserve QBU Hot Water Pusher Mechanical Seal
Answers from our engineering team on installation, compatibility and integration.
01What is the QBU seal designed for?
It is a single pusher seal designed for hot water services, including moderate-pressure boiler feed and circulation pumps. Lube Groove technology in the carbon face allows it to run in conditions where cooling water is unavailable or where energy savings are wanted.
02Does the QBU need external cooling?
Not up to 170 °C (340 °F) with a Plan 11 flush. For services up to 204 °C (400 °F) a Plan 21 cooling flush is added, which requires no change to the seal hardware.
03What are the QBU operating limits?
Pressure up to 17.6 bar (255 psi), a minimum vapor pressure margin of 2 bar (30 psi), speed from 750 rpm up to 20 m/s (65 fps) and shaft sizes from 12.7 to 139.7 mm (0.500 to 5.500 inch).
04Is the QBU interchangeable with QB seals?
The datasheet states that parts are interchangeable with conventional QB seals, which reduces inventory and speeds repairs. We recommend confirming the exact seal size and arrangement against your pump before ordering.
05What materials does the QBU use?
316 stainless steel metal parts, a carbon rotating face, a silicon carbide stationary face, fluoroelastomer, perfluoroelastomer or EPR gaskets and Alloy C-276 springs.
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