Pipeline

A couple of years ago I did a post about pipeline surge relief, which highlighted a liquid pipeline surge relief technical guide developed by Emerson's Daniel business.

Brady McKay, in the Daniel Canada organization sent me a great write up on the technical details of a surge relief system on which he's been working with a leading pipeline company. For those in the pipeline business, the workings of surge relief systems are probably obvious. For the rest of us, it may be interesting.

Surge pressure is the rapid change in pipeline pressure from the change in liquid flow rate. These pressure surges travel at sonic velocities through the pipeline and can range from 1100 ft/sec (333 m/sec) to more than 4000 ft/sec (1212 m/sec). The rate is a function of the liquid's compressibility and the content of entrained, dissolved gas. Greater compressibility translates into lower velocities. Stabilized crude oil has a velocity around 3300 ft/sec (1000 m/sec) during surge conditions.

So what causes these sonic pressure waves? Here are three examples: starting/stopping a pump, rapidly closing an emergency shutdown (ESD) valve or motor operated valve (MOV), and slamming shut action on a non-return (check) valve. What happens in these cases is that the pipeline flow drops rapidly. For those who recall their physics, the column of liquid continues under its own momentum (p=mv) that leaves behind a low-pressure region.

Eventually, the momentum is overcome by the opposing force of a static head, which accelerates the liquid column back toward the pumping station. This reversal causes the non-return/check valve to close. This process creates another pressure surge. How large this pressure will be is a function of the initial flow rate, the static head, pipe length, material, and friction inside the pipe.

Without sufficient surge protection in place, problems that can develop include:

1. Axial separation of flanges
2. Pipe fatigue at weld joints
3. Longitudinal pipe splits
4. Pumps knocked out of alignment
5. Severe damage to piping and piping supports
6. Damage to specialized components such as loading arms, hoses, filters, bellows etc.

A surge relief system must be extremely fast acting, on the order of less than 50msec. A liquid surge valve needs to open quickly to respond to the initial pressure rise. It then needs to close in direct response to the pressure decay on the inlet side of the surge valve. In a typical surge relief system, the relieved flow is dumped into a large storage vessel and then later returned to the pipeline when everything returns to steady state.

Brady's write up made great sense to me and I hope this summary has helped you. If you're already an expert in pipeline pressure surge, please share your wisdom in the comments.

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Last week I did a post about pipeline surge pressure relief and a technical guide about this written by Emerson's Daniel business. They are known for gas and liquid fiscal flow measurement solutions for the oil and gas industry.

I received a nice follow up note from Dave Seiler about a Latin American refiner who was fighting turbine meter maintenance problems due to large concentrations of foreign materials in the pipeline liquid flow. The problem was so acute that they actually had to install two meters in parallel so they could switch between meters while the other was being maintained.

The refinery engineers worked with the local Daniel team to replace the turbine meters with a 6-inch liquid ultrasonic flow meter. These do not have moving parts, unlike the turbine meters, which were being impacted by the particulates in the flow.

I didn't know much about the ultrasonic technology in flow applications, so I googled around and found a Hydrocarbon Processing magazine article reprint, Use liquid ultrasonic meters for custody transfer, in the Daniel area of the EmersonProcess.com website.

Dave is a co-author of this paper. The article does a great job of simplifying how the ultrasonic technology works. It also includes the math on how the ultrasonic flow measurement works.

My analogy, fresh from a rafting trip down the Guadalupe River, is to imagine that you're floating down the river with an ultrasonic transducer on one bank, and another on the other bank a little further downstream. Ultrasonic pulses are sent between the two transducers in each direction. The pulse traveling across the river from the upstream one to the downstream one will obviously travel faster since it's going across the river with the current. And of course, the reverse is true; it takes longer to travel across the river going upstream against the current. With the formulas in the article and enough perseverance, you can calculate the river's flow rate from these time differences. For the 3D world of pipe flow, the authors' explain:

The resulting time difference is proportional to the fluid velocity passing through the meter spool. Single and multiple acoustic paths can be used to measure fluid velocity. Multipath meters tend to be more accurate since they collect velocity information at several points in the flow profile.

Now back to the story... after the installation of an ultrasonic flow meter, the refiners saw that the meter was reporting low flow rates when the product in the pipe switched between gasoline and diesel.

The local Daniel service technicians collected maintenance logs using their Customer Ultrasonic Interface software (CUI) and sent it to the support team in Houston for detailed analysis. The team verified that the meter was working correctly for both liquids. They deduced that the flow was being diverted somehow during the transmix, or product switchover, where both products are flowing through the pipe until the switchover has been completed. This was possible because of the meters ability to accurately measure both flow rate and speed of sound of the liquid passing through the meter with extremely high accuracy.

The refiner verified that this is what indeed was happening where this transmix was being routed away through a smaller pipeline for further reprocessing. With the age of the refinery and the retirement of experienced operators, the current operators had not been able to see this transmix operation occurring in their process. The refinery engineers were impressed that the team in Houston could deduce this from their analysis of the data.

The refinery engineers involved in this project are presenting a workshop at this year's Emerson Exchange in late September. If you face similar challenges, you might want to catch this one.

I use a service, WatchThatPage, to track changes to various pages around Emerson Process Management. It sends me an email when any page in a list of pages I have created has changed. I use this as one of my sources for the posts I create. This helps me keep track of changes in non-RSS enabled pages. For those who don't use RSS (really simple syndication), here's some resources on how it makes your information quest more efficient.

Late last week I received an email notifying me of a change to the Daniel liquid pipeline surge relief technical guide. I caught up with Dave Seiler to ask about this application and some of the challenges process manufacturers with high-pressure pipelines face. Pipeline operators and those with high-pressure pipelines are quite aware of the potential damage that can occur if a pressure surge occurs.

Dave noted that over-pressurization of a pipeline is commonly caused by sudden changes in liquid velocity. This may occur when a pump starts or stops or a valve opens or closes. When a pressure rise occurs above normal operating pressure, it's very important to analyze the rate of the pressure rise to determine the proper size and type of valve required.

Dave described line blockage as the most serious pipeline issue. To mitigate this condition, pipeline design includes valve interlocking logic and clear operating procedures. As noted in the technical guide:

...pressure is contained must have some form of pressure relief, which is often mandated and regulated by local authorities. The design of such systems is dependent on a complex range of factors including, but not limited to, the potential for pressure increases, the volumes which must be passed by the pressure relief equipment in operation and the capacity of the system to contain pressures.

This guide describes applications you may have in your facility. On application is a pilot operated pressure relief valve used for pump protection duty and for similar applications where pressure relief is required to maintain pressure at a given set point. Another application might have exceptionally fast response times that require gas-loaded systems. These are described:

The basic valve is the balanced piston design. Nitrogen gas is used to pressurize the valve piston to keep it in the closed position. The valve incorporates an integral oil reservoir mounted on the external surface of the cylinder head, which upon installation is partially filled with a light oil. Gas under pressure is applied to the reservoir.

Other applications described include surge relief valve closed position and open position. I found the pictures like this one help make the text easily understandable.

If you have high-pressure pipelines in your process, take a look at this guide and see how it might help you.