August 2018

Process Control and Instrumentation

Adapting instrumentation to the needs of refineries

Oil refineries provide a smorgasbord of challenging applications for instrumentation engineers and technicians.

Carlson, D., Cychosz, D., Emerson Automation Solutions

Oil refineries provide a smorgasbord of challenging applications for instrumentation engineers and technicians. Safety is always at the forefront of every activity in a refinery due to these potentially hazardous conditions:

  • Flammable and explosive—The very nature of hydrocarbons
  • Hot—Converting oil into higher-value products takes heat
  • Corrosive—High total acid number (TAN) crude oils, hydrogen sulfide (H2S), alkylation acids, etc.
  • Viscous—Crude oil can contain residual content with very high boiling points.

Every type of challenge is not present in every unit, but they emerge in different places and in different combinations. Due to the sheer scale of the oil and gas industry, instrumentation suppliers work hard to develop solutions to these problems. Solving a critical problem can translate into widespread end-user benefits, since a given solution often works in many locations with today’s commonality of applications.

FIG. 1. A basic DP transmitter is one of the most versatile field instruments, and a mid-size refinery can have hundreds of them installed.
FIG. 1. A basic DP transmitter is one of the most versatile field instruments, and a mid-size refinery can have hundreds of them installed.

Due to the efforts of instrumentation engineers working innovatively with responsive automation solution providers, the toolbox for solving measurement problems is constantly growing. One of the sectors that has witnessed a significant improvement is differential pressure (DP) transmitters.

The basic DP transmitter (FIG. 1) is the “Swiss army knife” of instruments. It can be used for simple pressure measurements, along with DP, but it is also the most common way of measuring flow and is a frequent choice for level. This is especially the case in refinery environments, as using a few simple but clever accessories can extend the capabilities of a basic DP transmitter to handle the toughest challenges.

Too hot to handle

Within a refinery, high-temperature processes are basic to cracking and distillation, with operating temperatures in the 315°C–375°C (600°F–700°F) range being very common. Pressure instruments cannot withstand such heat with direct contact to process fluids, so ways must be found to transmit pressure without also transmitting heat. Uncompressible fluids with very high boiling points make this practical, since they can send pressure down an impulse line or capillary tube to the DP transmitter at a safe distance. This approach works, but is often defeated not by heat, but by cold.

A fill fluid that is incapable of boiling at 400°C (750°F) tends to thicken at more typical ambient temperatures. If the viscosity of the fluid in the impulse line gets to be too high, its ability to transmit pressure to the transmitter slows or can stop entirely. This is particularly troublesome during times when a cold front may move into an area, bringing ice and snow, no matter how briefly.

FIG. 2. Using two fill fluids optimized for different temperatures can extend the temperature range of a DP transmitter in a variety of applications. The cut-away section on the right uses the high-temperature fill fluid in the capillary tube. It pressurizes the intermediate diaphragm, which captures the low-temperature fill fluid that extends to the actual sensor diaphragm.
FIG. 2. Using two fill fluids optimized for different temperatures can extend the temperature range of a DP transmitter in a variety of applications. The cut-away section on the right uses the high-temperature fill fluid in the capillary tube. It pressurizes the intermediate diaphragm, which captures the low-temperature fill fluid that extends to the actual sensor diaphragm.

This problem can be solved by using two fill fluids (FIG. 2). The high-temperature fluid is used in the part exposed to the process. It bears the brunt of the heat and stays warm enough to avoid thickening. It sends the pressure value via an intermediate diaphragm to a second fluid designed for lower-temperature operation. This second and less-viscous fluid spans the remaining distance to the transmitter, and is unaffected by lower temperatures, even below freezing. This extends the temperature range of a DP transmitter without the need for heat tracing, and also greatly reduces the lag time in pressure readings as compared to other approaches.

This temperature range capability can be useful for any DP application—pressure, level or flow.

Calculating flow from pressure. Placing an obstruction, such as an orifice plate, in the path of fluid flow in a pipe causes a pressure differential in which the square root is proportional to the volume of fluid moving past the obstruction. Using a DP transmitter to measure the pressure drop can provide data from which the volumetric flow can be calculated. Adding a temperature and static pressure reading and known density characteristics can help convert a volume reading to mass flow. This capability has been recognized and utilized for many decades, and its ability to be used on a practical basis makes it the most popular method for measuring flow, particularly in refinery applications.

This fact still leaves the question: How well does DP for flow work in more extreme applications? We must first examine two long-standing problems: hot and clog-prone product flows, as often experienced in a crude oil distillation unit. In this unit, the crude is heated to a temperature around 345°C (650°F) and is then sent into the atmospheric fractionator column to separate it into its various fractions. These are heavy flows, usually requiring a pipe diameter of 12 in. or larger. Crude oil at this stage of its processing tends to carry impurities capable of clogging equipment.

FIG. 3. A wedge flow metering sensor, as shown in this cutaway, provides the necessary pressure drop and is both wear- and clog-resistant.
FIG. 3. A wedge flow metering sensor, as shown in this cutaway, provides the necessary pressure drop and is both wear- and clog-resistant.

With all these elements in mind, a reliable solution is a wedge primary element with remote seals (FIG. 3), which creates a reliable DP flow reading while being resistant to wear or plugging and reducing pressure loss. When mounted in a horizontal pipe run, the wedge cuts in from the side to keep an unobstructed flow path on the top and bottom, so there are no places for particulates or entrained gases to accumulate and impact measurement reliability.

Typical configurations place flanged taps on either side of the wedge element, although multiple sets of taps can be fitted for safety applications requiring redundant measurements. Since crude oil distillation is a high-temperature application, the impulse lines running to the DP transmitter often use a sealed system with multiple fill fluids to allow measurement of the high-temperature fluid, while avoiding the cost and maintenance challenges that are present with heat tracing.

DP for level on the distillation tower. Remaining with the distillation column example for a moment, think about another serious safety-related concern: flooding. There have been numerous safety incidents in refineries where a distillation column was unable to perform its function of separating vapors and, instead, filled with liquid, sometimes with catastrophic results. A very reliable level measurement is paramount to detect when this is happening.

Using a DP transmitter is an excellent choice, since it can be outfitted with a multiple-fluid temperature extender. To compensate for pressure inside the column, the low side of the DP transmitter can connect to a tap from the headspace at the top of the column. This creates a new set of challenges related to the best method for this connection, which has been treated as its own topic in many articles.

FIG. 4. Using a second transmitter for the headspace pressure reading, instead of using an impulse line, avoids many potential level measurement challenges.
FIG. 4. Using a second transmitter for the headspace pressure reading, instead of using an impulse line, avoids many potential level measurement challenges.

One approach growing in popularity is the idea of using a second pressure transmitter (FIG. 4) to read the headspace pressure, and then sending the reading to the transmitter at the bottom electronically. This action eliminates the long impulse line and can provide more information about the process, namely the headspace pressure. It also eliminates all the problems and expenses associated with impulse lines, including heat tracing.

Since level in this context is very much a safety issue, DP transmitters normally become part of the larger safety instrumented system (SIS), and three units are frequently installed to provide a two-out-of-three voting scheme. Typically, each of these units uses an identical configuration, and each can be outfitted with a multiple-fluid temperature extender.

Larger questions of DP flow. While DP flow metering applications are very common, the technique is not without its drawbacks. Examining them individually, we can see how vendors have found ways to mitigate problems:

  • Pressure drop—This basic measurement concept requires creating a pressure drop in the line, thus restricting flow, reducing available process pressure and creating a potential place for clogging. Traditional wisdom dictates that achieving the maximum accuracy and turndown range means creating the largest pressure drop. Fortunately, the accuracy of today’s DP transmitters is better than in years past, making it possible to get good flow readings with less pressure loss. The basic problem remains, but technology has mitigated the effect to a large extent.
  • Long, straight pipe sections—DP flow meters are affected by upstream flow disturbances, and therefore often require relatively long, straight and smooth pipe sections upstream and downstream from the primary element. Standards specify as many as 44 different pipe diameters upstream and five different diameters downstream to achieve maximum accuracy. This can make mounting a DP flowmeter in complex and congested piping a challenge. On new installations, additional cost can be incurred when piping runs are specified only to ensure accurate measurement.
         The design of the primary element is the main concern. The traditional round-hole-in-a-disk orifice plate is the most susceptible to this problem, but it is also one of the most common designs. In many respects, the requirement for straight piping relates more to the diameter of the restricting orifice than the pipe diameter. Using multiple holes (FIG. 5) rather than just one can alleviate the problem by significantly reducing the straight pipe requirement while maintaining high measurement accuracy and repeatability.
  • Impulse lines—Using DP to measure flow requires impulse lines between the transmitter and both sides of the primary element. The design and construction of the impulse lines have a major influence on the success of the installation. If poorly executed, they can be prone to many types of problems, including clogging, freezing, and slugs of gas or liquid. In refinery contexts, they are governed by strict piping requirements that specify welding techniques, shutoff valves, etc.
FIG. 5. The presence of four holes in the primary element, rather than just one, changes the turbulence characteristics, reducing the need for a long, straight pipe section.
FIG. 5. The presence of four holes in the primary element, rather than just one, changes the turbulence characteristics, reducing the need for a long, straight pipe section.

In critical applications, impulse lines are normally all welded and include gate valves on the high and low sides to isolate the transmitter. These allow the transmitter to be removed without shutting down the process. Bleeding ports are also included to clear gas slugs trapped in the lines.

Ports are placed close to the transmitter that can be opened to allow mounting of line rodding devices. These devices are mounted while the isolation valves are closed. Once the valves are reopened, the cleanout tool can be extended through the valve and all the way into the main pipe. This permits complete clearing of the impulse lines without any operational interruption.

Combining design developments

Various instrumentation vendors have put together different DP flowmeter designs in an effort to solve specific application challenges. Those aiming particularly at refinery environments fulfill piping requirements for design and construction, while providing sizes and configurations for the types of equipment found in refinery and petrochemical production units. For example, the DP flowmeter shown in FIG. 6 was assembled using the following elements:

FIG. 6. This flowmeter assembly is designed for difficult refinery operations, while providing a high degree of precision and reliability.
FIG. 6. This flowmeter assembly is designed for difficult refinery operations, while providing a high degree of precision and reliability.
  • A basic multivariable DP transmitter is combined with a preassembled spool section ready to mount in the application.
  • The impulse lines are kept short, and the DP transmitter is close-coupled to minimize the potential for plugging.
  • The primary element uses four holes instead of one to reduce the overall length between flanges, making it easier to fit into existing piping.
  • The overall construction is welded stainless steel, in keeping with piping requirements, and the entire unit has been leak-tested.
  • The gate-type isolation valves are also selected and welded according to piping requirements.
  • Threaded clean-out ports align with the impulse lines, so they can be rodded out while the unit is in operation.
  • Smaller isolation and bypass valves are built into the manifold, along with impulse-line bleeding ports.
  • The temperature sensor/transmitter assembly sends its information to the main DP transmitter, which can report the value to the automation host system. When the fluid temperature is combined with the DP volumetric flowrate and the known fluid density, the transmitter can also provide a mass flow measurement.
  • The DP transmitter can also provide static line
    pressure values to enhance the mass flow measurement without an additional pipe penetration.
  • Smart-instrument functions built into the transmitter provide a wide range of basic and advanced diagnostic information.
  • Although not designed specifically for high-temperature applications, a setup such as this can typically handle fluids up to 315°C (600°F) or higher, depending on the application and installation.

This product represents a combination of proven technologies optimized for the expected environment. It is built to provide a long and reliable life in refining environments, and the electronic components can be replaced on the fly without shutting down the line, if needed.

Instrumentation is key for control. Effective process control and safety in a refinery or petrochemical plant depend on effective instrumentation. This instrumentation provides the eyes and ears into a process, letting operators know what is happening inside the pipes and vessels. DP transmitters capture much of the data necessary to keep production units working efficiently and safely. HP

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