Manifolds Extend the Utility of Differential Pressure Transmitters
Adding a manifold simplifies mounting and maintenance when using differential pressure for flow or level, or when measuring pressure directly.
Within the world of process manufacturing, field instrumentation takes on many forms to measure variables such as temperature, flow, level and pressure — with the last parameter being the most common. This is largely because pressure instruments are used to measure flow and level in addition to conventional pressure readings. In fact, differential-pressure (DP) instruments are the most common way of measuring flow. While such versatility is advantageous, it presents challenges related to mounting complexity.
Consider a basic pressure gauge. It has a single inlet, typically a male pipe thread. It provides a gauge pressure reading, meaning the reading it shows is the process compared to atmospheric pressure. (Some specialized gauges provide an absolute pressure reading, or the process compared to an absolute vacuum, but these are rare.) To measure the pressure of liquid or gas in a vessel or pipe, the gauge is simply screwed into a threaded hole (figure 1).
Such a gauge provides a simple application with a simple solution. Straightforward accessories allow simple augmentations such as a way to isolate the gauge from the process fluid, or a siphon to handle condensate when measuring steam pressure.
Mechanical gauges have been largely replaced by electronic pressure transmitters, but connections to the process are still required. For a basic reading, a tube or pipe to the transmitter will suffice. While these approaches work for simple applications, pressure transmitters can be used in all sorts of ways and often need complex mounting approaches.
FIGURE 2. Plant-built differential-pressure flowmeter configurations can be effective, but they also tend to be maintenance intensive and easy to damage. Long impulse lines must be insulated or heat-traced in cold climates.
Many pressure transmitters are used in differential-pressure applications such as for measuring flow or level. In such uses, two inlets are needed to connect to the two points in the process the transmitter is comparing. For most applications, one point is always the higher pressure, so differential-pressure transmitters are designed with a high input and low input. If a gauge-pressure reading is needed, the low side is simply left open to atmosphere.
Manifolds Simplify Piping for Flow Metering
Many early electronic differential-pressure transmitter designs used a cylindrical assembly to support the actual measuring diaphragms in a biplanar arrangement. Configured like a bass drum, the flat round ends were the diaphragms, and the sensor elements were inside between the diaphragms. Signals from the sensors were sent to the electronic equipment in the attached housing for processing.
This configuration had no simple way to attach process connections, so a two-part flange bolted on from the outside like a clamp was designed and applied. This approach made it easier for designers to provide many connection sizes and positions while only changing or rearranging the flange parts.
As the numbers of electronic pressure transmitters began to grow, especially in flow-metering applications, users pressed vendors for different inlet configurations to satisfy more complex applications. One frequent complaint addressed the complexity of impulse-line configurations (figure 2) with demands for multiple fittings and tubing sections. If other accessories such as shut-off valves or isolators had to be added, they created additional connections where leaks could form. The early biplanar clamping flanges separated into two parts, making the impulse lines and accessories easy to displace during disassembly. These maintenance headaches caused users to look for better solutions.
The early biplanar designs have been phased out of many vendors’ product lines in favor of improved diaphragm and manifold configurations. For example, coplanar designs provide better protection for the diaphragms and avoid the necessity of a two-piece flange configuration. Current manifold designs simplify the inlet connection interface and provide a more firm and positive place to bolt the assembly. Manifolds can solve many application challenges while addressing the basic needs of mounting security and plumbing simplification.
FIGURE 3. Some flanges incorporate multiple options, combining threaded inlets with a machined-flat face to provide effective sealing. Some effectively duplicate the hole positions used with the flange-pair setups of older biplanar transmitters. This allows the transmitter to be installed using the existing impulse piping.
Modern transmitter/manifold combinations are designed for versatility and user convenience. Many configurations with threaded or flange-faced inlets are available to accept piping from any direction (figure 3). These options provide the means to make all process connections via the manifold. If the transmitter needs to be removed, it can be unbolted without disturbing any of the impulse piping. This basic concept of ease of assembly and maintenance is the primary advantage of manifolds.
Current manifold designs can be as simple or complex as the application requires. Three basic configurations are used:
- A basic gauge or absolute-pressure transmitter.
- A conventional differential-pressure transmitter manifold.
- An integral manifold.
The basic gauge or absolute-pressure transmitter typically uses a manifold with a single threaded inlet on the bottom. This provides isolation from the process fluid.
A conventional differential-pressure transmitter manifold is offered in many configurations, but all typically have two threaded inlets for high and low process connections. They are used in traditional plant-designed flowmeter applications. Alternatively, a flanged face can be used for attaching process flange adapters. Such a manifold provides isolation from the process fluid on both high and low lines. It may include threaded mounting holes.
An integral manifold can also be used with a differential-pressure transmitter in a flowmeter application (figure 4). An integral manifold bolts directly to the transmitter, eliminating the need for a transmitter flange.
The inlet holes on a manifold are either threaded or machined. When threaded, a pipe or fitting can be screwed in. When the face is machined flat and smooth, a matching flange can be bolted to it and sealed with a gasket or O-ring. With a machined face type of configuration, it is possible to remove the transmitter without breaking any pipe connections if the process connection can be shut off and the assembly isolated. All the piping can be welded if desired. The face-mounting option is used in many configurations.
The face-mounted option is especially useful for flowmeter applications. Traditional differential-pressure flow measurement configurations use impulse lines from the taps on either side of the restriction device — typically an orifice plate or a pitot tube — to convey high and low pressure readings to the transmitter. These impulse lines normally require multiple fittings, and often they end up being leak prone.
Modern designs replace the impulse piping with a small number of cast or welded components with a matching interface to the differential-pressure manifold. With all the elements bolted together, the assembly is robust, compact and leakproof. No fragile tubing is used to connect the two main components. Freezing issues are reduced because less surface area is exposed where heat can be dissipated.
Manifolds can be outfitted with valves built into the block itself. These can be as simple as individual shutoffs on the inlets, or they can be as complex as a full block-and-bleed configuration for critical process fluids. These valves can be particularly useful in high pressure differential-pressure measurements.
One of the trickiest commissioning jobs is installing a differential-pressure transmitter in an application where the two lines are at high pressure but there is little differential between the two. The sensor must be sensitive enough to measure the differential accurately, but it also must withstand the high line pressure on both sides. The worst situation is during test or calibration if a technician mistakenly allows only one side to become pressurized. This can ultimately damage the sensor. Three- and five-way valve configurations can open an equalization passage between the two sides so neither can be pressurized alone. When both sides are stabilized, the equalizing valve can be carefully closed so the pressure can reach its normal levels without any damage to the sensor.
Even the plugs used to close off the unused ports on a differential-pressure manifold can serve an additional purpose. They can be outfitted with small valves to bleed the impulse lines and displace air or liquid without breaking the seal on the plug itself. Alternately, they can be used as a calibration port to pressurize the high and low sides before opening the sensor to the process.
FIGURE 4. When a flowmeter is made as a dedicated unit including an integral manifold, it is possible to avoid many of the problems associated with impulse lines.
While manifolds are increasingly common, the extent of offerings from different manufacturers varies. Relatively few offer a comprehensive selection, and interchangeability is hit-or-miss. There has been some standardization of designs following IEC 61518 and ANSI MSS SP-99-2016a, but other configurations are unique to specific suppliers.
Some designs are interchangeable based on overall dimensions, hole spacing, diameters, threads and so on, allowing the transmitter from one manufacturer to bolt directly to the manifold of another. This is sometimes the case with inlet flanges designed for one manifold to mate with another as well. For example, 54 mm spacing between inlets is a common dimension for differential-pressure manifolds.
This interchangeability allows flexibility when a user is considering an upgrade. If a transmitter needs to be replaced, reviewing the dimensional charts of a few vendors often reveals more options than expected. It may be possible to add a transmitter with a higher degree of accuracy or more diagnostic capabilities without needing to touch the process connections at all. The new transmitter can simply bolt in place without any hardware modification.
Many users have gotten used to the idea of building a flowmeter using a pressure transmitter and manifold assemblies from a range of component parts. Technicians doing this kind of assembly must be conscientious, using correct seals, threaded fasteners and other specialized items. All the bolts, nuts and washers must be sized and placed correctly and tightened to correct torque specifications to ensure good sealing and avoid distortion of any components. Naturally, this assembly and testing takes time and costs money.
For a company installing one or two such assemblies per month, this is a minor expense. In cases where these transmitters are components of a larger product, or where many are being installed as part of a plant upgrade, assembly time can become a major consideration. Some vendors offer assembly and configuration services as part of the initial purchase. While these services are not free, the total installed cost may be lower, especially when considering the costs of doing this work in-house.
Factory assembly typically includes leak testing of all connections, allowing the field installation process to go much faster. Just as factory calibration is more accurate than most users can perform on a maintenance shop bench, assembly by trained technicians can be more reliable.
Pressure transmitters are used in many process plants to help monitor and control production. They can perform a range of functions and function in creative implementations. Manifolds can be used to address any challenges by simplifying installation and reducing maintenance time and costs over the product lifecycle. PC