Advanced level instrumentation improves performance and safety
Most industrial safety incidents, related to tank overfilling, involve level instrumentation failures, which cause operators to continue filling a tank when it has reached its capacity. These types of incidents often involve multiple problems with safety devices failing to shut down pumping action.
This article concentrates on more common situations where an overfilling incident begins to take shape but is stopped by the safety system before it is too late. These avoid the worst outcomes such as fires, explosions and environmental messes, but still put the plant through the disruptive actions of emergency valves closing and pumps shutting down, which are problematic in their own right. The common causal element is usually a failure of the primary level instrumentation.
Such safety incidents are the most obvious problem related to poor level measurement performance. There are also more subtle effects, such as:
- Ineffective feedstock and finished product inventory management
- Production interruptions if feedstocks run out unexpectedly
- Inefficient use of storage resources
Obsolete level instrumentation can create a host of hidden dangers for a plant related to inaccurate readings and outright failures. No plant should depend on its safety instrumented systems to make up for poorly performing primary instrumentation.
Legacy measuring approaches
For many years, owners of the large storage tanks typically found in manufacturing plants, terminals and other tank farms depended on mechanical approaches such as float-and-tape setups. These work when they are well maintained, but like any piece of machinery, they can fail when their pulleys and rollers do not turn. Similarly, many safety sensors use floats and other moving parts that can also jam or break down.
More sophisticated plants might use differential pressure (DP) instruments with sealed or vented tanks. DP offers significant advantages over mechanical alternatives when used in ways that utilize its capabilities well and take its limitations into account. The following sections concentrate on specific considerations associated with DP level applications and how the technology has been refined for better performance.
Creating highly effective DP level installations
The first area is understanding how liquid density affects a level reading. For example, oil products have lower density than water, so a pressure reading calibrated in inches of water will require correction for oil. Naturally, any plant using DP level measurements should understand this and adjust accordingly, including factoring in temperature as it often causes variations in density.
The more complex situation is where tanks are sealed and can experience internal system pressure above or even below atmosphere. If a large tank holding water and vented to atmosphere shows a pressure reading on a simple pressure gauge of 15 pounds per square inch (psi), it is correct to assume there is 34.6 feet of water above the gauge. If the tank is not vented and it is pressurized to 5 psi, the reading will change drastically. The offset can be corrected by deducting the system pressure, but the easier approach is using a DP gauge and tying the low side to the headspace. This makes the reading self-correcting. Again, it is a basic concept, but implementing it requires careful evaluation.
Attaching a gauge or electronic DP transmitter to a tank requires a physical connection between the sensor and tank contents or headspace. The traditional method uses impulse lines for the connections, which allows the contents to reach a bourdon tube or internal diaphragm. Of course, this makes the gauge or transmitter and the impulse lines part of the tank’s containment, which can become a maintenance headache with potential for leakage.
The solution is to install a remote diaphragm seal at the tank connection, which isolates the contents while sending pressure readings to the gauge or transmitter via a sealed line filled with silicone oil or other fluid. This wet leg approach eliminates most of the maintenance problems, but can introduce concerns of its own, particularly with tall tanks. If a diaphragm and wet leg are used for the headspace side, the weight of the fill fluid creates its own pressure, which affects the headspace reading.
There are methods to correct for this effect (see Figure 1A and B), but these can introduce additional complications, particularly with equipment outdoors and exposed to the elements. Fill fluids are designed to work best in specific temperature ranges. Those intended to accommodate high temperatures tend to become viscous when they get cold. For example, a transmitter with fill fluid that is selected to accommodate warm contents inside the tank but is exposed to a cold outside environment may not read the headspace pressure correctly due to excessive viscosity in the filled line coming from the top. Where such conditions are possible, the line should be heat traced, but this adds complexity and maintenance concerns.
Heat tracing is difficult to install and expensive to operate. Steam heat tracing requires adding steam traps, and if the system shuts down for whatever reason during cold weather, it can be harmed by condensate freezing and corresponding damage to pipes. Electric heat tracing is arguably easier to work with, but may not be permitted in hazardous areas. It also requires its own controller and can take a long time to warm up to the required temperature. Either way, heat-tracing systems consume a great deal of energy while adding limited value to operations.
Avoiding insidious wiring problems
Even if the tank connections and impulse lines are working flawlessly, level measurements can be corrupted by instrument wiring problems. Getting a signal from a DP transmitter on a tank to the automation host system may involve sending it hundreds of feet, with many wiring terminations in between. A broken or short circuit will cut off the signal entirely, but more subtle problems can interfere in ways that are not drastic enough to be immediately recognizable but are still able to mislead operators. Here are two examples:
Leakage current: A partial short circuit (see Figure 3) allows current to “leak” from one side to the other due to moisture or corrosion, which might change the signal level by a few mA. For example, if the actual signal is 15 mA with leakage of 3 mA, the automation system will believe the reading is 12 mA and show an incorrect value to the operators.
New advanced transmitters, such as Emerson’s Rosemount 3051S Pressure Transmitter, have loop integrity diagnostics, which perform tests for these conditions automatically, monitoring electrical integrity on a continuous basis to ensure accurate measurements. Should current leakage or voltage sag be detected, the 3051S can send alarms to operators and maintenance as directed.
Minimizing downsides
It is possible to optimize how technologies are applied to minimize downsides and avoid the need for tradeoffs in performance. For example:
- Advanced transmitters with sophisticated diagnostics can perform loop verification. These ensure that data is collected accurately and sent to the automation host system without any corruption, so operators have a clear picture of tank contents.
- The problems associated with impulse lines and heat tracing can be solved by eliminating the impulse lines. An electronic remote sensor can read headspace pressure electronically (see Figure 1C) and send the value to the main transmitter at the bottom. The sensor on each transmitter uses a diaphragm to avoid potential containment mishaps and maintenance problems. The two transmitters are matched to deliver a high degree of accuracy and can send a headspace process pressure reading as a secondary variable.
Maximizing capabilities
The following is an example of how these beneficial factors can all come together. The MillerCoors brewery in Milwaukee has 26 large fermentation tanks, which receive wort from the brewhouse (see Figure 5). The company uses DP level transmitters and has experimented with various methods to solve the impulse line from the headspace problem. The brewery is now using a Rosemount 3051S Electronic Remote Sensor (ERS) System and has realized several significant benefits:
- The headspace impulse line is gone entirely, eliminating seasonal temperature problems.
- The upper sensor sends a pressure reading for the tank headspace in addition to correcting the level reading.
- The headspace pressure reading can trigger a pressure-relief system, avoiding blowing out a rupture disk.
- With the headspace pressure reading, operators can determine when fermentation has begun due to CO2 causing a pressure increase.
- Once CO2 pressure has reached a threshold, collection can begin to capture it for use in the bottling process, reducing the amount food-grade CO2 purchased.
- The volume of CO2 produced and changes in density of the wort, which can be detected by the level transmitter, indicate the amount of fermentation happening and when it is complete for the batch.
- With a positive indication of completed fermentation, time spent in the fermenters has been reduced, increasing throughput by 400,000 barrels per year.
Megan Wiens is a global pressure product engineer for Emerson in Shakopee, Minnesota. She is responsible for advanced diagnostic capabilities across the Rosemount pressure portfolio. In this role, she works to implement product solutions that improve plant safety, increase process efficiency, and enhance process insight. Wiens holds a B.S. in chemical engineering from the University of North Dakota.
Nicole Meidl is a product manager for Emerson in Shakopee, Minnesota. She specializes in Rosemount DP level products, but in her five years with Emerson, she has managed Rosemount pressure transmitters, MultiVariable transmitters and Electronic Remote Sensor systems. Meidl has a B.S. in mechanical engineering and is currently pursuing an MBA from the University of St. Thomas in St. Paul, Minnesota.
