Seals cope with low vapour pressure margins

The QBQ LZ provides high-reliability sealing with low emissions, claiming a new standard in high-reliability sealing for pumps in light hydrocarbon service at low vapour pressure margins.

The Flowserve QBQ LZ contacting-face mechanical seal features precision face topography and the company's patented wavy face seal design.

The QBQ LZ provides high-reliability sealing with low emissions, and sets a new standard as the high-reliability sealing solution for pumps in light hydrocarbon service at low vapour pressure margins.

Optimised for service with light hydrocarbons of specific gravity between 0.40 and 0.60, the QBQ LZ is designed for use in refineries and the general hydrocarbon processing industry pumping ethane, ethylene, propane, propylene, butane and other related liquids.

Light hydrocarbons present unique challenges for these facilities because of their natural tendency to flash from liquid to vapour at ambient pressure and temperature conditions.

Vaporisation leads to dry-running of seal faces and premature seal failure, which results in the double failure of not meeting emissions and reliability objectives.

"Equipment reliability is a key plant metric today", says Mark Fallek, Vice President of Marketing, Flowserve Flow Solutions.

"Refineries are constantly looking to improve their best-practice mean time between repair (MTBR) and maximise plant productivity".

"At the same time, environmental legislation around the world is increasingly taking aim at reducing hydrocarbon and volatile process emissions to the atmosphere", adds Fallek.

"Refineries seeking to increase their reliability, MTBR and emissions control have a new weapon in the QBQ LZ".

The QBQ LZ, a new model of the industry-standard Flowserve QBQ seal, addresses the problem of vaporisation in light hydrocarbon service at low vapour pressure margins by altering temperature and pressure dynamics on the seal faces.

Using precision face topography technology, smooth, low-amplitude patented wave patterns are laser-machined onto the seal face to create a stable hydrodynamic effect that changes the pressure profile on the seal faces and reduces friction and contact loads without increasing leakage.

Producing low contact loads means less temperature rise on the seal faces, which enables the QBQ LZ to operate in light hydrocarbon services with lower vapour pressure margins - between 0.34 and 3.40bar - than any other contacting seal face design on the market.

With less sensitivity to vapour pressure margin, the QBQ LZ also remains reliable even when upsets or variable process conditions arise.

The QBQ LZ reduced contact loading also minimises start-up torque and the face damage associated with start-ups and shutdowns, dramatically extending seal life and MTBR.

The patented wavy face design of the QBQ LZ produced by Flowserve laser machining process features precise, repeatable patterns and smooth surfaces across the seal faces.

Smooth surfaces are critical to the success of the QBQ LZ contacting-face design, as they prevent the entrapment of solids and enable the QBQ LZ to achieve its unique dynamic effects.

These dynamics occur as fluid pressurises on the wave peaks of the QBQ LZ at pressures several times higher than seal chamber pressures, driving vapour back into liquid.

Vaporisation is pushed as close as possible to the inner diameter (ID) of the seal, leaving the QBQ LZ seal faces primarily wet with stable flashing and little seal-generated heat.

This prevents dry-running, enables the QBQ LZ to operate at extremely low vapour margins, and promotes long and reliable operation.

The QBQ LZ features a wave topography specially engineered for low vapour margin light hydrocarbons.

The design generates a fraction of the heat associated with competing contacting-face seals and reduces cooling requirements.

An advantage to contacting-face seals like the QBQ and QBQ LZ is that they offer much lower emissions than lift-off seals.

The QBQ LZ uses a thin-layer wet seal similar to the QBQ's to keep emissions low.

The unique technology of the QBQ LZ satisfies all API 682/ISO 21049 design and qualification test requirements for single- and dual-arrangement seals.

Because it is based on the standard and widely used Flowserve QBQ seal, the QBZ LZ is easily retrofitted by simply changing the rotating and stationary seal faces with minimal inventory impact and quick implementation times.

The QBQ LZ offers the same basic construction as the QBQ, including a rugged cartridge design and multi-port flush injection.

Beyond its application for vapour pressure margins between 0.34 and 3.40bar, the QBQ LZ can also be used for high-pressure pumps above 20.7bar at any vapour margin.

The QBQ LZ operating range includes pressures from 0 to 51.7bar, temperatures between -40 and +203C, speeds between 6.1 and 23m/s, and sizes from 44.4 to 130mm.

On double-ended, double-suction pumps, where premature vaporisation is a common concern, the bidirectional QBQ LZ can be installed on either end without pump modifications or other measures normally required to boost vapour pressure margin.

"Bad actor" pumps that consistently cause equipment downtime due to vaporisation can realise a dramatic improvement in MTBR after upgrading to the QBQ LZ.

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