Carver’s Model 850 Series Pump Solves Cavitation and Priming Issues in Vapor Liquid Separator Application used for Environmental Remediation
February 25th, 2022
After trying multiple pump types, and 6 pump failures due to cavitation, Paul Michalski of 212 Environmental was desperate and determined to find the right pump for his Vapor Liquid Knockout Tank.
Upon asking around, and scouring the internet, Paul found a reference to Carver Pump on an engineering forum and called Carver Pump. Paul explained the issues he’d had with other centrifugal pumps, and carefully explained how the system worked to Derek King, Carver’s Central Region Sales Manager (563) 203-0037 firstname.lastname@example.org. Before hanging up the phone on the initial call, Carver Pump sized and selected a Carver Model 850 Horizontal Filtrate Pump perfectly suited for this type of low NPSH pumping.
Skeptical of Carver’s claims, Derek scheduled a trip to the site to help the team at 212 Environmental set up their controls and come up with a parts list necessary for the install. Carver Pump made recommendations, and when the pump was delivered, the installation went off without a hitch.
About the Application
212 Environmental operates an RTO (Regenerative Thermal Oxidizer) system in Hartford, IL. This system is used to extract and treat petroleum hydrocarbons present in soil vapor and groundwater recovered from the vapor collection system. The system also collects rainwater infiltrate following precipitation events. The extraction wells connect to horizontal conveyance lines through a series of 33 system control vaults.
System vacuum is induced using one of two 150-hp belt-driven positive displacement blowers. Typically, a single blower is operated, providing a maximum flowrate of 1,560 standard cubic feet per minute (scfm) at a vacuum of 195 inches of water (in-H2O). The operating flowrate and vacuum maintained by the blowers varies over time and is directly related to conditions within the operating extraction wells including groundwater elevations and soil gas permeability.
Recovered vapors are passed through an 820-gallon knockout tank to separate rainwater infiltrate and groundwater from the soil vapor. The rainwater infiltrate and groundwater are transmitted through an oil-water separator before being stored in three 16,340-gallon double-walled above ground storage tanks. Extracted water is transported to a wastewater treatment facility. Oily water recovered via the separator is stored and periodically transferred as a non-hazardous liquid for subsequent treatment and disposal at an off-site facility.
Volatile petroleum hydrocarbons recovered within soil vapor and groundwater are treated within an Anguil Systems, Inc. Model 50 regenerative oxidizer.
The Challenge With Vapor Liquid Separators
The pump issues that Paul was seeing were taking place at the knockout tank. The knockout tank is designed for a flowrate of approximately 2,900 scfm at 160 in-H2O vacuum. The extracted vapors, groundwater, and rainwater infiltrate enter the knockout tank through a 16-inch diameter inlet located near the bottom of the tank. Vapors are diverted upward and pass through a wire mesh demister filter which removes the liquid droplets. With a much higher gas-liquid ratio, the vapor leaves the separation equipment through a 16-inch diameter outlet.
Meanwhile, the separated liquid (groundwater and rainwater infiltrate) accumulates at the bottom of the knockout tank and are transferred to an oil-water separator. This transfer is where the model 850 pump comes in. The issue with using a standard centrifugal pump is that the vacuum being pulled on the tank makes for a low Net Positive Suction Head available; or NPSHa. The other issue is that a typical centrifugal pump has to be sized perfectly and continuously tweaked to accommodate varying flows. A standard centrifugal pump cannot lose prime or run dry.
Pump Cavitation Causing Damage
The problem with low NPSHa is that it causes pump cavitation in centrifugal pumps.
Cavitation happens when the static pressure of a liquid is below that of its vapor pressure. This causes the water to boil, creating steam bubbles inside the pump. Once these air bubbles implode, this causes vibration. Though one implosion may be small, thousands can chip away at the pump metal – this is how cavitation damages the pump, and how cavitation affects pump operations.
Cavitation can occur anywhere in the pump line. There is suction cavitation, where the pressure at the suction of the pump is too low, and there is discharge cavitation, where a high discharge pressure causes the fluid to circulate rather than exit the line.
Pump Priming Issues
In typical applications with a centrifugal pump, there is a consistent flow rate. This is why after performing several calculations, the correct pump size can be found to let the pump function properly and avoid cavitation. Once the priming liquids fill a correct-sized pump, it can run without a high risk of cavitation.
However, in the vapor liquid separator, the vacuum created a very low pressure at the pump impeller. Plus, since the flow is inconsistent, sizing the pump properly is extremely difficult.
These are the reasons why cavitation already caused 6 pump failures. Because even if a positive displacement priming pump was used to help with the low NPSHa, the inconsistent flow rate may still cause cavitation.
These two, seemingly difficult parameters made the Carver 850 Series the perfect selection.
Why a Carver model 850?
The model 850 and its sister series, the 855 are rugged in design, dependable in nature, and have been proven in some of the earth’s harshest environments. Capable of operating in vacuum conditions up to 26” Hg (353 in-H2O), these pumps are well suited for use on vapor liquid separators or liquid knockout tanks. A vent port allows pumping to resume even after short periods of interrupted flow. Meaning, the pump can empty the tank, continue to run until the tank begins to fill again, and then re-prime itself and continue pumping, all without hurting the pump. This assures continuous pumping, from a trickle to full capacity. This capability is well suited for liquid knockout tanks where the incoming fluid can vary in amount. Horizontal Vacuum Filtrate Pump | 850 | Carver Pump
With a specially designed semi-open impeller and a variety of material choices ranging from cast iron, to duplex stainless steel and rubber lined design, the 850 series is ideal for abrasives and light slurry duty, passing small solids that would clog an enclosed impeller.
The 850 is available as either a close coupled or frame-mounted unit with an option for side-mounted motor with belt drive. The front and back pull-out design allows for easy maintenance. And the pump is available with a number of shaft sealing options.
10 Months After Installation
After Paul and the 212 Environmental Team had the pumps installed, and after the pumps had ran for 9-10 months, Derek took another trip down to the site to take some photos, and make sure everything was still operating as it should with the pump and get some more information about the system for this article.
During that visit Paul Michalski of 212 Environmental said, “When you write about this project, talk about Carver’s customer service, because we’ve never had this type of service from a pump manufacturer.” he continued, “I was skeptical, after 6 other pump failures, I needed this one to work, but I was skeptical. (Carver Pump)… coming down here to the site and conferencing in Carver’s Inside Sales Manager to help create a parts list for install and discuss the controls portion of the project was invaluable.”
It was clear that Paul and the 212 Environmental Team had found their solution. By trusting Carver Pump, they were finally able to find a pump that worked perfectly for their application and not, as had happened so many times before, see yet another pump fail.