Water Storage Systems

Transcription

1 Water Storage Systems Water Storage Tanks Types: Gravity or Elevated Ground Storage Hydropneumatic Water Storage Tanks Ground Storage Hydropneumatic Elevated Storage 1

2 Diurnal Flow Pattern Peak Demand Plant Flow Fill Tank Average Daily Demand Empty Tank 12M 12N 12M Ave. 24 Hour Period Schematic Illustrating the Benefits of a Storage Tank Elevated Storage Tanks Slide 6 2

3 Elevated Tanks Stabilize distribution system pressures Provide water to meet peak demands Keeps pumps from cycling and operating in efficient ranges Fluctuating Levels in an Elevated Storage Tank Average Flow Tank Empty Well or High Service Pumps Tank Full Peak Flow Pumps are used to supplement system pressure during peak demands & refill storage tank during other times. Advantages Offered by Elevated Tanks Less variation in pressure Available water for fire fighting Storage to meet peak demands Allows use of lower capacity wells Cycling of well pumps is reduced Wells can be better matched to average water demand High service pumps and the treatment plant can operate more efficiently 3

4 Maintenance Considerations for Elevated Water Storage Tanks Check for Intrusion of Water Secure Tank Site from unauthorized access Clean Tank yearly recommended (DEP 5-yr. required!) Ensure that overflow structures are working, secure and properly drained Inspect Structures for Stability, Blockages and Surface and Internal Corrosion. Standpipes Elevated Tank and Standpipe Comparison Used to provide pressure head to the distribution system. Large storage capacity Shallow tanks with large diameter are preferred over deep one with small diameters. Less pressure drop in elevated tanks. Pressure = Water Elevation (ft) * Water Density (lb/ft ) * ( 1 ft2/ 144 in2) Where: Water Density = 62.4 lb/ft 3 4

5 Ground Storage Tanks Slide 13 Ground Storage Used for storing large amounts of water. New Tanks must be Covered! Not under pressure uses transfer pumps to pressurize or pump to elevated tank. Sometimes has aerators on top for waters that require it. Hydropneumatic Tanks Must be built to ASME standards Slide 15 5

6 Hydropneumatic Tanks Under 120 gallon PST 2000 tanks allowed by FDEP Slide 16 Hydropneumatic Tanks Combination of compressed air pressure to force water into the distribution system when pump is off Slide 17 Types of Hydropneumatic Tanks Conventional Floating Wafer Flexible Separators Flexible diaphragm Bladder type Floating Wafer 6

7 Operating Considerations for Conventional Hydropneumatic Tanks Must have air to operate correctly Must have a pressure relief valve Need a sight glass to observe level in tank Must have By-pass piping for repairs Valve for flushing Air volume control Scheduled maintenance program Components of Conventional Hydropneumatic System Components Function Air Volume Control - Control amount of air in tank Relief Valve - Prevent excessive high pressure Pressure Gauge - Monitor water/air pressure Motor Controls - Control cut-in and cut out Level Controls - Regulate high/low water levels Compressor - Replenishes Air Conventional Hydropneumatic Tank Schematic 7

8 Hydropneumatic Tank Side View Overall Length Diameter PRV Compressor Hydropneumatic Tank New must meet ASME Stds & interior coating must meet NSF 61 requirements Gauge Tank Level Sight Glass Tank Drain Tank Bypass Line with Isolation Valve (not shown), normally closed From Well Hydropneumatic Tank End View ASME Name Plate Access Manway max. size allowed by ASME Code 24 min. if possible per FDEP Elastomeric gasket / sheet between tank & support Foundation New Hydropneumatic Tanks as required by FDEP Rule (20), FAC and Recommended Standards for Water Works (a.k.a. Ten States Standards.) Sizing Considerations for a Conventional Hydropneumatic Tank Capacity of the wells and pumps should be at least ten times the average daily consumption. Gross volume of the tank, in gallons should be at least ten times the capacity of the largest pump, rated in gpm. Sizing must consider the need for disinfectant contact time. Operating pressure is generally sized to operate between 40 and 60 psi The pressure pump should not cycle more than 6 to 8 times per hour 8

9 Air Compressor Maintenance Considerations Regularly scheduled maintenance: Clean or replace filters Drain condensate frequently NSF approved lubrication Hydro-Tank Sanitary Deficiencies Exterior staining does NOT always mean corrosion or possible contamination Hydro-Tank Sanitary Deficiencies Pitting, spalling, cracks, holes, coating deterioration 9

10 Hydro-Tank Sanitary Deficiencies Undercoat is still intact No Pitting, spalling, cracks, holes, coating deterioration here Hydropneumatic Tank Failures Metal skin under continual tension Crack rips at hundreds to thousands of miles / hour You will hear it AFTER failure occurs! You will most likely not have a chance to react Hydropneumatic Tank Failures This water system called about their hydropneumatic tank They had just welded a plate over a pinhole leak Luckily 2-weeks later when the tank blew out, no one was in the immediate area and no extensive damage was done to other structures. This type of repair significantly compromises the tank & shorten its life Instead of strengthening the tank, the welds increased metal stresses 10

11 Hydropneumatic Tank Failures 11