Pumps, valves and seals selection: Part Two

Schubert & Salzer

At the heart of a Schubert & Salzer Control Systems’ sliding gate valve lie two slotted discs sliding against each other, and forming a seal. One sealing disc is permanently fixed in the valve body and the other is movable. The selection of the ideal material pairing for these sealing discs depends on many factors. In addition to the friction coefficient, which has a direct effect on the level of the force needed to actuate the valve, there are other aspects such as leakage rate, chemical resistance, suitability for high differential pressures, edge stability as well as operation in cavitation, which are all critical parameters in the choice of material or coating for the sealing discs. Moreover, the desired service life and replacement costs have to be considered.

The conventional materials for the movable sealing disc based on carbon or stainless steel with a hard alloy based on cobalt and chromium (Stellite®) are now supplemented by a diamond like coating known as SFC (slide friction coating).

The SFC is applied to a stainless steel sealing disc and brings together the stability of the stainless steel base body, the very low coefficient of friction of the carbon and the extreme robustness of the diamond like coating layer. Thus, movable sealing discs coated with SFC can withstand high differential pressures while, at the same time, the actuation forces for moving the sealing discs remain extremely low. This makes it possible to have smaller, and hence much more cost effective valve actuators. Insensitivity to pressure shocks, operation near the closing point with improved edge stability and the relatively low leakage rate round off the advantages of this sliding disc coating.

The SFC coating not only improves the frictional properties of the base body but also forms an enveloping hard layer around the base material and protects it from chemical and mechanical attacks by the medium or by contaminants. Sliding gate valves with an SFC coated sealing disc are ideal not just for shutting off and controlling high differential pressures at low actuation forces but they also present an easy solution for dealing with carrier fluids, saturated steam, acids and caustic solutions. Furthermore, a sealing disc with an SFC coating can be used for media temperatures from – 60 °C to + 300 °C.

SKF

A new seal design improves upon traditional, commonly available T-seals and spring backup seals, giving operating equipment manufacturers (OEMs) and end users a more robust high pressure seal for applications such as downhole tools, well control mechanisms and subsea equipment. The locking T-seal design can reduce installation time and damage, improve equipment reliability in harsh environments and extend service life.

T-seals

T-seals have been widely used in oil and gas equipment. The generally ‘T’ shaped section of the rubber part provides a stable seal, while the flexible rubber protrusions keep the rigid plastic backup rings in contact with mating hardware, thereby preventing extrusion of the rubber into metal clearances under high pressure. These backup rings have an angled split for installation and can be difficult to keep oriented while assembling components. Without careful attention or special installation tools, they may be partially out of the seal groove and be severed by installation of the mating components or they can fall completely out of the groove without being observed by the installer. In either case the rubber seal is prone to fail without support from the backup ring. The loose backup ring material could cause problems elsewhere in the system.

Spring backup seals

Spring backup seals were designed to be easier in installation and assembly. The backup rings are replaced by metal springs molded in to the rubber at each side. When adequately bonded to the rubber, these springs prevent flow of rubber toward the hardware clearance gaps at high pressure. If this bond fails and the rubber detaches from the spring, the seal may fail or the spring may fall out on assembly or disassembly, potentially causing trouble elsewhere in the system. Spring seals are also not recommended in dynamic reciprocating motion because the metal spring can cause damage to sliding surface.

Locking T-seals

A new design, the patented SKF Locking T-Seal has detents which snap into place when installed and mechanically lock the anti extrusion rings in the proper orientation, enabling easy installation without damage to the seal or other system components. The T-seal can be used in dynamic reciprocating motion, as well as high pressure static connections.

Weir Minerals

Weir Minerals Lewis Pumps has developed an international reputation for its products in the oil and gas industry in sulfur and sulfuric acid applications over the past 120 years. Located in Saint Louis, Missouri, USA, the company has put pumps and valves into service in more than 120 countries around the world. There have been many changes throughout the company’s history, and Weir Minerals Lewis Pumps continues to evolve to meet the demands of the market.

Pump sizes and materials

Offering a range of vertical pumps in sizes from 1 in. (30 mm) to 18 in. (450 mm) and a line of valves for acid service ranging from in. (10 mm) to 30 in. (750 mm), Weir Minerals Lewis Pumps keeps plants operating successfully around the world. Lewis® Pumps vertical chemical pumps offer flow rates up to 13 000 gal./min (3000 m³/hr) and lengths up to 468 in. (11 880 mm).

The company draws on a rich background in alloy development and pump application to provide premium equipment for high specific gravity and corrosive services. Many pumps and valves incorporate Lewmet®, a proprietary alloy developed by Weir Minerals Lewis Pumps that provides maximum resistance to corrosion, erosion, abrasion and galling. With a product line that includes vertical submerged pumps, vertical externally mounted pumps, cantilever shafted pumps, horizontal pumps, axial flow pumps, valves and specialty application items, Weir Minerals Lewis Pumps has a wide range of products to suit a variety of applications in the oil and gas industry.

Industry knowledge and experience

Weir Minerals Lewis Pumps takes great pride in having knowledgeable staff that can provide continuous support to meet customers’ requirements. Frequent visits by sales engineers to customer facilities around the world enable the company to develop a better understanding of customers’ application requirements. The workforce of men and women who work diligently to produce products and serve customers is the most valuable resource at Weir Minerals Lewis Pumps. The vast knowledge possessed by the company’s work force is a cornerstone of the its success.

Recognised as a world leader for pumps and valves in the oil and gas industry, Weir Minerals Lewis Pumps has experienced 120 years of growth and industry recognition. With a dedication to providing quality products, the company is committed to supporting the industry for another 120 years.

WEKA

WEKA cryogenic control and shut off valves and components are based on what is now more than 40 years of uninterrupted experience in supplying helium and hydrogen plants around the world, in bulk and research applications. Unique design features of WEKA cryogenic valves include large control variability that allows straight process flow design without additional bypass solutions. The high flow capacity of valve relative to its nominal size and double functions of control and shut off reduces the number of valves needed. This allows an even more compact valve box design and low heat load which increases plant efficiency. The proven robust design with high mean time between failures (MTBF) has distinguished the company.

Cryogenic valves in numerous references around the world under critical service conditions in helium and hydrogen cryogenic plants. In July 2013, RasGas Company Ltd., Qatar announced the successful completion of Helium 2 plant at Ras Laffan refinery. This helium liquefier is a horizontal vacuum isolated drum box including absorbers, plate fine heat exchangers, static gas bearing expansion turbines and most of the WEKA’s cryogenic valves for exact process control. The plant is expected to produce approximately 37 million m³/y of helium gas when fully operational. Together with the Helium 1 plant, operational since 2005, a combined annual production of approximately 57 million m³ of helium gas will be produced. The two plants together make up the world’s biggest helium refining facility currently in service and will meet approximately 28% of the current total global liquid helium demand.

WEKA AG supplied all cryogenic shut off and control valves in dimensions DN15 (1/2 in.) to DN150 (in.), fully designed and tested in accordance with ASME and EN construction codes. WEKA AG is proud to be part of the world’s biggest helium liquefier currently in service in Qatar, which will help to solve the world’s helium shortage in the near future.

Zwick

Zwick is one of the most respected manufacturers of metal seated triple offset butterfly valves, Series TRI-CON, in the world with more than 20 years of experience. Mr Hans Zwick, owner and managing director, developed a large range in the past. The Series TRI-CON is available in sizes 2 in. - 72 in. and pressure classes from PN 10 – PN 160 or ASME Class 150 - 900. Mr Zwick also designed the very successful derivatives of the Series TRI-CON, the metal seated check valve, TRI-CHECK, and the Double-Block and Bleed version, TRI-BLOCK in this time. Besides this range, company Zwick is very strong in special applications with several customised features.

Bearing failures are the highest root cause of failure in triple offset butterfly valves (TOV). This issue is directly attributed to the need to have metal bearings with very tight tolerances when accepting the shaft diameter.

An example of efforts being made to find a solution to this issue is Hans Zwick’s mechanical design that insures that both the inside diameter (ID) and outside diameter (OD) of the bearings are protected, and would also not violate the Fire-Safe design criteria. The design incorporates three rings of die form graphoil packing ring at the very end of the bearing ID, and then three rings of the same on the OD, both are captured by machined edges and loaded by the packing load which keeps the graphoil rings from flattening out. This patented design is called the sealed bearing feature. Besides the fact that this design prevents any media from entering into the bearing cavity, the advantageous design to minimise shaft deflection of the company's standard bearings can be kept in the special design. Another plus is the fact that every standard Zwick valve can be upgraded with the sealed bearing design. Besides the protection of the bearing in this design, it functions as an additional seal to the packing and minimises fugitive emissions to a minimum that makes this bearing system also very attractive for applications in ethylene oxide or propylene oxide. The patented Sealed Bearing has been proven for more than 12 years in many applications such as butadiene, styrene, sulfur, sulfur tail gas, EO, PO, high viscosity applications like thermal oil and acid gas.

Adapted for the web by Emma McAleavey.

Read the article online at: https://www.oilfieldtechnology.com/special-reports/01072014/pumps_valves_and_seals_part2_814/