Beyond 150°f, plastic tubing can pigtail, stretch and break. Stainless steel eductor piping goes deep with low maintenance. A Top-head-drive piston pumps are ideal to rescue hot zones
There are many reasons why landfill operators scratch their heads when landfill gas production falls short of projections or earlier output. Why are some gas wells watering in, allowing fluid to take the place of gas, when others are performing well at the same site?
It could be that overaggressive pumping is roiling the down-well liquid, helping to create carbonate buildup, foaming or silting, clogging the well. That forces frequent pump maintenance and promotes wear, decreasing well efficiency.
The solution here could be controlled low-flow pumping or installing new pumps with more adaptable pump technologies – more about that later. Or it simply might be that the materials of construction of the well and the pumps used in these wells is no longer up to the job. This is particularly true if your site qualifies as an Elevated Temperature Landfill (ETLF), contains elevated-temp zones or is experiencing higher heat trends in some areas.
It is now accepted that deep in the center of many older landfills, in larger, wetter, mature zones beyond the reach of oxygen, the exothermic melding of aged materials is increasing temperatures, endangering not only the effectiveness of individual site pumps but the viability of those site systems as well, including LFG.
Traditional landfill pumps are not built for temperatures beyond 150°F / 66°C, and as heat rises, traditional air pumps struggle. Mechanical parts become encrusted and begin to break down. The system falters.
More than the Pump
But an air pump’s internal parts are not the only components that suffer as a zone heats.
A surprising yet significant cause of air-pump system dysfunction is that downhole air tubes or hoses – which have been a smart choice at normal temperatures (and shallower depths) because of their light weight and flexibility – become elastic or brittle over time at higher temps (and greater depths), altering their shape and strength.
Even if a pump itself appears to be operating, the eductor/riser tubing begins to lose its integrity at 150°F. Hose connections begin to leak. The hotter it gets, the more fragile downhole hoses or tubing become. Enough heat and the flow stops.
Yet preventative intervention – during the period between recognition of rising temps and before a zone would devolve into a full-blown ET state – can slow the progression and or even rescue a failing system (if the pumps can operate at higher temps), ET site engineers say.
The questions a manager asks are obvious: What do I have to do, when do I have to do it and how much will it cost?
One Solution – Hard Pipe
At some of the most difficult or threatening ET zones, and even in zones where heat is not an issue but greater pumping depths are required, early-adapting site managers are returning to an old acquaintance – using stainless-steel hard pipe as the fluid eductor.
Steel piping gave way to hose/tubing years ago in part to accommodate the needs of popular compressed-air pumps, whose maintenance requires pulling the pump to clean, repair and replace internal pump components, including three-tube bundles that are necessarily immersed in the landfill fluids they pump.
The ability of steel to maintaining integrity to temps of 750°F / 400°C allows heat-resistant pumps, particularly top-head-drive piston pumps, to operate efficiently and for long periods in ET wells, avoiding common downhole pump failures and premature wear, while increasing well pumping production. One early-adapting landfill site engineer reports flow-rate increases of 17 percent or more after replacing eductor tubing with common 1¼-ich steel pipe.
And as engineers go deeper in the landfill to 200 feet / 60 meter or more, they may find that stainless pipe, unlike flexible tubing, has the structural strength to withstand increased weight pressure.
But most importantly, steel piping is beginning to demonstrate the ability to revive heat-zone viability and potentially rescue LFG and leachate systems from failure when paired with top-head-drive, positive-displacement piston pumps.
Replacing downhole eductor tubing with hard pipe might appear daunting. Recent installations demonstrate it is not. Hard pipe installation is common in water-well and oil-well operations, using readily available and inexpensive pump- and pipe-hoisting tools that are field-friendly.
Wellfield technicians and a truck-mounted, pipe-hoisting winch can install steel hard pipe in 10-foot segments in roughly the time it normally takes to pull and reinstall an air-drive pump. Credit the water-well tools to make hard-pipe installation easier and safer. And using stainless drain-plug female NPT pipe coupling with each section pipe lowers the fluid weight of the hard pipe should it ever need to be pulled. When field techs are trained on the techniques, one site manager reports an install time of one hour (or less) for a 100-foot well.
Top-head-drive piston pumps, with mechanical drivers and power above the wellhead, are ideally suited to hard-pipe installations. A piston pump’s drive piston and drive rod are installed permanently from the surface. Downhole pump pulling is not required. Primary pump maintenance and service is done at the surface wellhead. Significantly, piston pump technology avoids the necessity of a three-tube/hose bundle immersed in harsh downhole landfill gas and liquid environments.
When and How Much?
If hard pipe answers the manager’s first question, numbers two and three are quick to follow.
When? If your thermometer reads 150°F or above and your LFG production is falling, ASAP is logical. The longer one waits, the greater the threat that the entire system will need to be replaced…once the zone temps subside.
Cooling depends on dewatering wells, deeply and consistently enough to allow lighter gas to replace heavier fluid, which retains heat. Without serious efforts to dewater, the zone could be lost. Solar, electric and pneumatic top-head-drive piston pumps are proven to maintain reliability even as popular air pumps struggle or fail beyond 150°F.
If your thermometer shows a trend toward increasing temperatures but has not yet reached 150°F, the risk is apparent. What some managers have learned is that preventative measures are paying off. How Much? One U.S. site estimates the cost of replacing downhole eductor tubing with 304 stainless-steel pipe, using the site’s technicians, to be in the range of $750 a well, including the piping and connectors. Of course, unit prices fall with larger orders.
The result? A revived and newly functioning LFG system.