Orifice Flow Meter Fouling Prevention

Orifice flow meters in fouling service have been a problem in delayed cokers and sulfur units since the beginning. I have been to many plants that have developed their own home grown solutions to this problem after years of trial and error. This article will quickly review some of the solutions that we have found effective at various levels. 

Some of the locations in the with fouling flow meters include. 

• Fresh feed
• Heater pass flow
• Heater charge pump minimum flow return
• Blowdown tower bottoms
• SRU Acid gas flow
• Claus reactor dP
• Molten Sulfur flow

Most new units are designed with wedge, ultrasonic, or Coriolis type flow meters to avoid some of the plugging issues with traditional orifice plate flow meters. While these are nice, many old units can not justify the expense to upgrade due to piping constraints or general corporate return on investment (ROI) hurdle rates. Even in some new units, these services have orifice flow meters because of "value engineering" during the design phase. Regardless of the reasons why your site might have this type of flow indication, there are ways to make it better. 

A high level list....

• Really good impulse line insulation and heat tracing
• Eccentric orifice plate
• Angled taps - free draining
• Tap pumping system
• Continuous tap purge
• Fully enclosed tap cleaning tool
• Auto-rodding tap cleaner

Let's break it down a bit further....

For a fluid which is solid at room temperature, it goes without saying that keeping it hot is mission critical for the flow indication to work. Make sure the taps and the impulse lines are kept hot. Ensure good contact between steam or electric tracing lines, remember air is a good insulator. If you are not sure how good a job the installation crew has done, use an IR camera to have a look at these lines for variations in temperature. 

Fouling tends to accumulate in the bottom of the pipes in horizontal runs, the exact place where we like to put orifice flow meters. Using an eccentric orifice plate increases the likelihood that the solids will pass through the opening in the plate. The orifice plate opening and pressure drop will need to be recalculated with an eccentric design but it's a simple fix. 

Orienting the impulse line taps at 2 and 10 o'clock can help minimize plugging. Typical installations of orifice plates in non-fouling applications is at 3 and 9 o'clock. We have even seen them at 6 o'clock but I have no idea what they were thinking there. Having a slight downward tilt will help keep the taps clear of solids. 

Speaking of plugged taps, there are a few ways to keep them clear that we mentioned earlier, a portable pumping system, continuous purge, or automatic rodding system. 

Using a portable glycerin or glycol hand pump is the go-to solution for many plants to push fouling solids and plugs of cold resid or sulfur back into the process stream. If the board operator is paying attention and reacts quickly enough to the alarm signal, heater trips could be avoided. But the outside operator needs to be quick. It's an effective solution but maybe not the best. 

Continuous purge is an option to keep a small flow pushing back into the process. In the coker, we use purge steam in most of the structure valves to keep them clean, why not here? It turns out that flow meters (most types, not just orifice plates) do not like 2-phase flow. So steam purge is out. Heavy Coker Gas Oil is a readily available, non-flashing fluid for this application. It works in some locations to keep the taps clear and is a fully functional solution. There is a draw back to this solution, the recycle flow cost. With the caveat that all systems are different, let's make some assumptions for the cost of this solution on a typical 4 drum, 2 furnace DCU with a 4 pass furnace and triply redundant flow indication per pass. If the purge is 0.1 gpm per tap, the total purge is 0.1 x 2 x 4 x 3 x 2 = 4.8gpm HCGO purge into the process. At a margin of 10 $/bbl margin on HCGO into the diesel pool, we have a cost of $600,685 per year. It is a fully functional solution but a costly one. 

Once the taps are plugged with solids, they must be mechanically cleaned out. The practice of cleaning orifice flow taps, bleeders, level gauge taps, pressure gauges, etc was performed historically (and presently in some plants) using all manner of metal implements like welding rod, #9 wire, or anything else approximately the right size and shape the operator can find on the unit. Using welding rod is not recommended and is unsafe. Tools like the Flow Boss from American Maintenance Systems are an effective way to rod out these small connections and restore them to function. They are a fully enclosed, pressure rated system to drill out the solids that pack into the small bore piping.

Recently, Clearguard, a company from Australia, has been gaining popularity with their Autorodder technology around the world. The small device mounts onto the taps and periodically pushes a small rod back through the tap to clean it out. Pretty simple, right? Typically the impulse lines only plug in the first few inches so an automated cleaning only makes sense. It works on impulse lines and vessel pressure or level taps. This relatively simple device could save the unit and the operators a bunch of headaches and upsets. 

Why is this important you might ask. Some ideas to consider are, 

• Prevent spurious heater tripping and fouling
• Prevent overfilling of the fractionator bottoms or surge drum
• Prevent catalyst contamination in the Claus reactors
• Optimize the unit recycle and conversion

In summary, there are a number solutions from the relatively cheap to complex which have been used to solve these problems in our industry. We hope any or all of these ideas can help your site. If you have orifice plates on your unit and have some effective solutions not listed here, we would love to hear about it for the benefit of the entire #Refcomm community. Please post them in the comments section of this article.