Hydropneumatic Tanks:

Hydropneumatic tanks are ASME (American Society of Mechanical Engineers) and non-ASME vessels that hold water and air under pressure. The tanks do not have a bladder and so air is in direct contact with the water. The compressed air creates a cushion that can absorb or apply pressure as needed.

Uses of Hydropneumatic Tanks:

Hydropneumatic tanks are particularly useful for providing on-demand pressurized water without the continuous need for a pump and are also very useful for water storage in many small water systems. By regulating system pressures, hydropneumatic tanks provide efficient water supply to quickly meet system demand.

This type of tank serves three main functions when used in water systems:
1. Delivers water within a predetermined pressure range so the well pump is not constantly running.
2. Stops a pump from starting up every time there is a slight demand for water from the distribution system.
3. Minimizes the occurrence of pressure surges, also known as water hammer.

Causes of Hydropneumatic Failures

Hydropneumatic tanks are very valuable and convenient but they require regular care and maintenance. The metal skin of pressure vessels are under constant tension. When corrosion, crack, or break occurs, it can rip through the metal skin at very high speeds, up to thousands of miles per hour, even under normal operating pressures. Pressure vessels are dynamic devices and so when they fail, they can fail catastrophically leading to major damage, injury, and potentially death (see Figures 1 and 2).

 

        

Fig 1: Hydropneumatic Tank Failure in CT          Fig 2: Hydropneumatic Tank Failure in FL

 

During the summer of 2016, a conventional 50 year old hydropneumatic storage tank failed under normal operating conditions at a community public water system in Connecticut (See before and after incident pictures in Figures 3 and 4). The tank failure resulted in a large explosion and the pump station was completely destroyed. Two hundred and twenty nine customers from this system were impacted by the tank failure, including numerous residential homes, a hotel and several local small businesses.

 Thankfully, the explosion occurred around 3:00 a.m. and no injuries, or loss of life, occurred. The distribution system was depressurized, but significant emergency measures were required to restore and sustain water service, including the use of a mobile hydropneumatic tank (see Figure 5). An analysis indicated that several factors contributed to the tank’s catastrophic failure including internal corrosion, age, and poor/uneven welding at the time of construction.

 

 


Observed Condition of Tank (6/19/16)                   Condition of Tank After failure (6/23/16)

Fig 3: Twin Hydropneumatic tanks before            Fig 4: Tanks after catastrophic failure

 

Fig 5: Mobile Hydropneumatic Tank Being Used In An Emergency In CT

Reports of other catastrophic hydropneumatic tank failures have also been documented recently.  Several tanks have failed in California in which one resulted in the death of a wastewater system operator in June 2011. Florida Rural Water Association has reported that seven tanks failed in Florida in recent years, two resulting in fatalities. In June 2018, an operator in Connecticut called to report a significant leak of a hydropneumatic tank that serves an apartment complex located at a pump house where the main production well is situated. The situation resulted in discolored water and loss of pressure, where significant emergency measures were required to restore and sustain water service at a high cost to the utility.

Given the above incidences, Connecticut Department of Public Health wish to alert owners of public water systems, including particularly small water systems, of the hazards concerning aging hydropneumatic and surge tanks and the need to develop a plan to assess and replace such tanks. Steps should be taken to ensure that these tanks are inspected regularly and repaired as and when needed. Regular preventative maintenance, along with annual and five-year inspections, should be scheduled.

Hydropneumatic Tank Asset Management

As of November 2018, there are approximately 297 active small community public water systems, each serving at most 1000 year-round residents in Connecticut, that have hydropneumatic tanks as part of their facilities. EPA estimates the useful service life of a well maintained hydropneumatic tank at 10 years as discussed in the Simple Tools for Effective Performance (STEP) guide Taking Stock of Your Water System: A Simple Asset Inventory for Very Small Drinking Water Systems.  A number of factors can affect how long the hydropneumatic tank can last, including routine service and proper maintenance, excessive use, and environmental conditions such as poor source water quality, soil quality and climate.

Pictures of some hydropneumatic tanks taken during routine sanitary surveys or other compliance site visits are shown below in Figures 6 - 9. 

 

      

Fig 6: Hydropneumatic Tank that needs cleaning     Fig 7: Housed tank that needs maintenance                       

 

           

Fig 8: Left Hydropneumatic; Right - Atmospheric    Fig 9: Abandoned tank with water on the floor 

What is DPH doing about Aging Hydropneumatic Tanks at Small Community Water Systems

https://www.cga.ct.gov/2018/ACT/pa/2018PA-00168-R00HB-05163-PA.htm

CT DPH is requiring, as detailed in DWS Circular Letter DWS2018-00XX, that all Small Community PWSs that serves at most 1000 year-round residents shall prepare a fiscal and asset management plan for all capital assets that comprise the system.  An assessment of all conventional (non-bladder) hydropneumatic and surge tanks shall be prioritized as part of the asset and fiscal management plan and completed by no later than May 2, 2019.  Small Community PWSs shall use the Hydropneumatic Tank Assessment Form to meet the requirements of Public Act No. 18-168.

Hydropneumatic Tank Replacement Options

If your hydropneumatic storage tank has outlived its useful service life, instead of spending funds on conducting an assessment, your PWS may wish to move straight to replacement or removal of the aging tank. With the advancements made with variable frequency drive controllers for well and booster pumps, many PWSs are opting to create constant pressure systems eliminating the need for hydropneumatic tanks. Please remember that any new water system projects must be submitted to the Department for review and approval prior to construction using the General Application Form.    

Additionally, some projects may be eligible for funding from the Drinking Water State Revolving Fund (DWSRF).  DWSRF is the Section’s low interest loan program. Community public water systems and non-profit non-community public water systems are eligible to apply for and receive a loan from the Drinking Water State Revolving Fund (DWSRF) for the replacement or elimination of an existing hydropneumatic tank. The DWSRF offers loans terms up to 20 years on loans over $100,000 and shorter term loans for lower cost projects. DWSRF interest rates are approximately half of market rate but no lower than 2%. Subsidized loans may be available to small public water systems serving fewer than 10,000 persons and to larger public water systems located in distressed communities. Eligible public water systems may apply for a DWSRF loan at any time but all new applications are required to undergo a public review process and be ready to proceed in order to qualify for funding.

Within the DWSRF, a Small Loan Program is available for obtaining loans under $100,000 with streamlined procedures for projects that eliminate hydropneumatic tanks and replace them with variable frequency drive pumping systems. The Small Loan Program projects is only available for projects that do not include the construction of new facilities, existing building alterations/additions, the use of heavy equipment for site work and where the hydropneumatic tank can be abandoned in place without removal. For more information on this loan program please visit the Drinking Water Section’s DWSRF Website or contact a representative from the DWSRF Program at (860) 509-7333.

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