Bidirectional valves
Bidirectional valves: technology, application and the right choice
The working principle of bidirectional valves
Bidirectional valves seal equally well regardless of which side the pressure comes from, which removes flow direction as a constraint on installation and operation. The term bidirectional valves is used broadly in the market, which is why it helps to define precisely what it means in your case. In practice bidirectional valves is rarely a standard question once medium and temperature move away from average. A triple offset design combines two geometric offsets with an inclined cone axis, so contact between disc and seat only occurs in the last few degrees of travel. That offset removes the rubbing movement across the seat, which is the main cause of wear in concentric butterfly valves. In an offset butterfly valve the centre of the disc does not sit in the centre of the flow, so the disc lifts away from the seat immediately on opening. That principle explains why bidirectional valves behaves differently from a standard execution under heavier duty. We regularly see that a small change in the execution of bidirectional valves makes a large difference to maintenance frequency. With bidirectional valves the seat is the component that determines performance, and the body is rarely the limiting factor. Both the seat and the sealing edge are metal, so temperature, steam, vapour and aggressive media cause no degradation of the seal. The result of that fourth offset is that sealing performs equally well in both flow directions, which is not a given with triple offset valves. Once bidirectional valves is correctly specified it disappears from the fault list, and that is exactly the intention.
Executions and materials for bidirectional valves
Many valve designs have a preferred direction, and their sealing performance drops noticeably when pressure comes from the other side. A correct execution of bidirectional valves accounts for real operating conditions and not only for design values. With bidirectional valves it pays to include phase changes in the medium, because those sharply increase the load on the seal. In systems with bypass circuits, recirculation or possible backflow, unidirectional sealing is a genuine operational risk. Since a controlled contact face is formed in the closed position, no run in period is required and the valve performs to specification from the first stroke. A quadruple offset construction adds a fourth offset, which distributes the load far more evenly around the full circumference of the seat. Because sealing is achieved by torque rather than by deformation of an elastomer, tightness is maintained over thousands of cycles. The quadruple offset geometry is one of the constructions that achieves true bidirectional metal seating. Maintenance is limited to packing and bearings, and because the seat is metal there is no liner to replace. Installation orientation is less critical on bidirectional designs, which prevents mistakes during field assembly. Operating conditions govern the service life of bidirectional valves far more strongly than the nominal figures on a datasheet. What weighs heaviest on bidirectional valves in practice is the combination of temperature and differential pressure, not either one alone.
Bidirectional valves in demanding installations
They are used in gas systems, steam bypass circuits, storage installations and anywhere flow direction can reverse. Offshore, weight saving counts, and at larger diameters a butterfly valve saves considerably compared with a ball valve. In power generation they sit in steam, condensate and flue gas lines where temperature is far beyond the range of soft seats. On project supply we record per position which execution of bidirectional valves was delivered, including materials and tests. On replacement we always check the existing installation dimensions for bidirectional valves, so the pipework can stay untouched. For bidirectional valves the sector determines which documentation and tests belong with the delivery. We see demand for bidirectional valves rising in hydrogen and ammonia projects, where tightness requirements are high. In technical gases and hydrogen applications the metal seal is decisive, because small molecule size and emission limits reinforce each other. Our market is project driven and international, and we deliver worldwide to installers, EPC contractors and end users. What defines us is that we would rather spend an extra half hour on the specification than ship the wrong valve that comes back two months later. In plants where bidirectional valves is applied, the emphasis is on reliability rather than lowest purchase price. In an existing plant bidirectional valves is often part of a wider question about availability and maintenance.
Choosing the right bidirectional valves
On bidirectional valves we check that the actuator is sized on torque at maximum differential pressure and not on an average value. Where a ball valve or gate valve becomes a problem through mass and face to face length, an offset butterfly valve often delivers the same tightness at a fraction of the weight. In selection we look first at medium and temperature, then at differential pressure, and only after that at body style and actuation. When replacing an existing gate valve we check face to face length and flange connection, so the pipework does not have to be modified. Where there is doubt about bidirectional valves we quote two executions side by side, with the trade offs stated explicitly. Standardising across several positions makes bidirectional valves simpler in maintenance and in spare parts management. Where bidirectional valves forms part of a safety function we record the fail position and required stroking time explicitly. In vacuum service a metal seated offset valve outperforms an elastomer seat, because there is no liner that can be drawn inwards. The disc can be reworked when worn, so the service life of the body is often many times that of the sealing components. If you have an existing tag plate or serial number, send it along, because that lets us work directly from the original configuration. The right choice around bidirectional valves starts with a full set of process data rather than with a request for a price. For bidirectional valves total cost of ownership weighs more than purchase price, certainly when replacement has to happen during a shutdown.
Ordering bidirectional valves from Dutch Valve Vision
For bidirectional valves you can come to Dutch Valve Vision for the selection, the delivery and the support afterwards. We work as a technical supplier rather than a catalogue reseller, which means we carry out the selection ourselves on the basis of medium, pressure, temperature and code requirements. We supply bidirectional valves as a loose product or as a fully assembled unit with actuator and accessories. We think along on the spare parts strategy for bidirectional valves, so critical positions stay covered. On bidirectional valves we come back with a proposal that explains why we recommend a particular execution. We keep spare parts and overhaul kits mapped per project, so that a breakdown does not start with a search from scratch. For shutdowns and turnarounds we look at accelerated routes, and in those cases we are honest up front about what is and is not achievable. We record which execution of bidirectional valves was delivered, so a follow up delivery or spare part matches exactly later on. A tag plate with project data belongs with bidirectional valves as standard where the project requires it. Material certificates and test reports are supplied with bidirectional valves, matched to what the project requires. Our specialists are happy to discuss bidirectional valves on the basis of your P&ID, your datasheet or simply a photograph of the situation. Call +31 (0)70-2210560 or send your specification to sales@dutchvalvevision.com and we will pick it up.
Frequently asked questions about bidirectional valves
What exactly does bidirectional valves mean?
In an offset butterfly valve the centre of the disc does not sit in the centre of the flow, so the disc lifts away from the seat immediately on opening. Because sealing is achieved by torque rather than by deformation of an elastomer, tightness is maintained over thousands of cycles. Both the seat and the sealing edge are metal, so temperature, steam, vapour and aggressive media cause no degradation of the seal. The result of that fourth offset is that sealing performs equally well in both flow directions, which is not a given with triple offset valves. The answer to this question on bidirectional valves changes as soon as the medium or the temperature changes. Our experience with bidirectional valves comes from projects in chemicals, power generation, technical gases and offshore. Anyone looking into bidirectional valves quickly runs into a choice that goes beyond picking a catalogue number. Alongside QUADAX and PRUSS we represent Pibiviesse, Schroeder, Striko, Malbranque, Daume, FluoroSeal and Guichon, so we can select the right brand for each project.
Which executions of bidirectional valves are available?
The disc can be reworked when worn, so the service life of the body is often many times that of the sealing components. Maintenance is limited to packing and bearings, and because the seat is metal there is no liner to replace. The quadruple offset geometry is one of the constructions that achieves true bidirectional metal seating. In systems with bypass circuits, recirculation or possible backflow, unidirectional sealing is a genuine operational risk. If you are unsure about this on bidirectional valves, we are glad to put the question alongside the actual process data. On bidirectional valves we prefer a well founded answer to a fast one that turns out to be wrong later. The balance between required tightness and available operating force is a recurring tension on bidirectional valves. Our customers work in chemicals, petrochemicals, pharmaceuticals, power generation, offshore and in hydrogen and ammonia projects.
Which materials are used for bidirectional valves?
Many valve designs have a preferred direction, and their sealing performance drops noticeably when pressure comes from the other side. In vacuum service a metal seated offset valve outperforms an elastomer seat, because there is no liner that can be drawn inwards. That offset removes the rubbing movement across the seat, which is the main cause of wear in concentric butterfly valves. Since a controlled contact face is formed in the closed position, no run in period is required and the valve performs to specification from the first stroke. For bidirectional valves a good specification up front is cheaper than a correction afterwards. In most projects bidirectional valves turns out to be less standard than it first appears. For bidirectional valves this means that construction and operating conditions cannot be judged separately. We think along from the basic specification through to the tag plate, and supply documentation that a buyer or inspector accepts without discussion.
What should I watch technically on bidirectional valves?
What should I watch technically on bidirectional valves?
Which applications call for bidirectional valves?
In technical gases and hydrogen applications the metal seal is decisive, because small molecule size and emission limits reinforce each other. Offshore, weight saving counts, and at larger diameters a butterfly valve saves considerably compared with a ball valve. In chemical and petrochemical plants these valves are used where both tightness and emission requirements carry serious weight. They are used in gas systems, steam bypass circuits, storage installations and anywhere flow direction can reverse. That makes bidirectional valves a subject where discussion up front pays for itself in operation. We see this come back regularly in enquiries about bidirectional valves, and the answer is rarely a single sentence. The application range of bidirectional valves runs from simple utility duty to heavy process plant, with a corresponding difference in execution. Dutch Valve Vision supplies special valves for special applications from its base in Monster, the Netherlands, with QUADAX and PRUSS as the core brands in the programme.
What determines the price and lead time of bidirectional valves?
For shutdowns and turnarounds we look at accelerated routes, and in those cases we are honest up front about what is and is not achievable. If you have an existing tag plate or serial number, send it along, because that lets us work directly from the original configuration. We come back with a proposal that explains why we recommend a particular execution, including the alternatives we considered. Lead times depend on the execution, because a standard size from a running series behaves very differently from a valve in an exotic alloy with NACE certification. This is one of the points on bidirectional valves where suppliers differ most clearly from one another. The answer on bidirectional valves depends strongly on operating conditions, which is why we always ask for those first. We handle an enquiry for bidirectional valves on its technical merits rather than as a price request that only produces a number. Requests can be submitted by phone on +31 (0)70-2210560, by e-mail at sales@dutchvalvevision.com or through the contact form on our website.
What about maintenance and spare parts on bidirectional valves?
Installation orientation is less critical on bidirectional designs, which prevents mistakes during field assembly. On repeat orders we record the configuration, so a follow up delivery is identical to the first and installation holds no surprises. We quote a realistic lead time rather than an optimistic one, because a missed delivery date costs more than an extra week in the planning. We keep spare parts and overhaul kits mapped per project, so that a breakdown does not start with a search from scratch. If you are unsure about this on bidirectional valves, we are glad to put the question alongside the actual process data. This is one of the points on bidirectional valves where suppliers differ most clearly from one another. We keep the configuration data for bidirectional valves on file, which saves a great deal of searching if a fault occurs later. Our market is project driven and international, and we deliver worldwide to installers, EPC contractors and end users.
How do I request a quotation for bidirectional valves?
For an accurate quotation we need the nominal diameter, the pressure class, the medium, the operating temperature and the required actuation. A P&ID, a datasheet or even a photograph of the existing installation often helps us faster than a long description. Please also state the connection standard, the required leakage class and any project requirements such as NACE, PED or ATEX. Documentation such as material certificates, test reports and declarations is supplied as standard, matched to what the project requires. We always explain why we arrive at a particular recommendation on bidirectional valves, so you can follow the reasoning yourself. On bidirectional valves we prefer a well founded answer to a fast one that turns out to be wrong later. For repeat orders of bidirectional valves we record the configuration, so a follow up delivery is identical to the first. Bring us your question at sales@dutchvalvevision.com or +31 (0)70-2210560, even when the specification is not finalised yet.