Guidelines for Designing Backflow Prevention Assembly Installations

Transcription

1 Guidelines fr Designing Backflw Preventin Assembly Installatins Supplement t the 1981 Crss Cnnectin Cntrl Manual - January 1992 Purpse The purpse f these guidelines is t augment and/r clarify thse guidelines utlined in the January 1981 Crss Cnnectin Cntrl manual. These guidelines reflect accepted design cnsideratins based n experience in implementing crss cnnectin cntrl prgrams and plicies set frth by the American Water Wrks Assciatin, Envirnmental Prtectin Agency, USC Fundatin fr Crss Cnnectin Cntrl and Hydraulic Research and state and lcal health departments. Pending revisins t the manual, these guidelines shuld clearly utline what an acceptable design and installatin cnstitutes. They are t be reasnably interpreted and will be updated as new design slutins and technlgies are ffered. General Installatin Details I. Clearances All duble check valve (DCV) and reduced pressure zne (RPZ) backflw preventin assemblies are designed fr in-line service and must be installed t prevent freezing, flding and mechanical damage with adequate space t facilitate maintenance and testing. Ideally, the installatin shuld nt require platfrms, ladders r lifts fr access. Adequate clearances frm flrs, ceilings and walls must be prvided t access the test ccks and t allw the repair and/r remval f the relief valve and check valves; as fllws: II. All assemblies shall be installed with a centerline height frm 30 inches t 60 inches abve the flr. Any installatin at a greater height shall be prvided with a fixed platfrm, a prtable scaffld r a lift meeting OSHA standards. All RPZ devices must have an 18 inch minimum clearance between the bttm f the relief valve and the flr t prevent submersin and prvide access fr servicing and relief valve. A minimum f 12 inches f clear space shall be maintained abve the assembly t allw fr servicing check valves and fr peratin f shut-ff valves. A minimum f 30 inches f clear space shall be maintained between the frnt side f the device and the nearest wall r bstructin. At least 8 inches clearance shuld be maintained frm the back side f the device t the nearest wall r bstructin. This clearance may need t be increased fr mdels that have side munted test ccks r relief valves that wuld be facing the back wall. Miscellaneus Cnsideratins All assemblies shall be adequately supprted and/r restrained t prevent lateral mvement. Pipe hangers, braces, saddles, stanchins, piers, etc., shuld be used t supprt the device and shuld be placed in a manner that will nt bstruct the functin f r access t the relief valve. Strainers are recmmended prir t each backflw preventin assembly n nn-fire fighting water lines. N strainer is t be used in a fire line withut the apprval f the Insurance Underwriters r the authrity having jurisdictin.

2 III. Drainage The assembly shuld be sized hydraulically, taking int accunt bth the vlume requirements f the service and the head lss f the assembly. The head lss f the assembly is nt necessarily directed prprtinal t flw. (Refer t the manufacturers head lss curves). Befre selectin and installatin, refer t manufacturers literature fr temperature ranges. All assemblies must be prtected frm freezing temperatures and if installed where temperatures will reach 100 degrees F r abve, a ht water type assembly must be used. Cnsult manufacturers specificatins fr recmmendatins. Thermal water expansin and/r water hammer dwnstream f the assembly can cause excessive pressure. T avid pssible damage t the system and assembly, use water hammer arresters, surge prtectrs r expansin tanks as apprpriate. All assemblies shuld be specified and installed with the manufacturer supplied resilient seated shut-ff valves integral t the assembly. Water lines shuld be thrughly flushed befre installing the assembly. Mst test failures n new installatins are the result f debris fuling ne f the check valves r the relief valve. All assemblies must be installed hrizntally unless they are specifically apprved fr vertical installatin. (Ref. Technical Reference PWS-14). Parallel installatins shuld be cnsidered at thse facilities where water service cannt be interrupted. Manifld installatins may als be used n any water line larger than 10 inches. Assemblies shall nt be installed in areas cntaining crrsive, txic r pisnus fumes r gases which culd render the assembly inperable r pse a safety hazard t persnnel. Because f the inherent design f a reduced pressure backflw assembly, fluctuating supply pressure n an extremely lw flw r static flw cnditin may cause nuisance dripping and ptential fuling f the assembly. While nt effective in all cases, the installatin f a sft seated check valve immediately ahead f the RPZ will ften hld the pressure cnstant t the assembly in times f fluctuating supply pressure. Where the distance between the water meter and the device is greater than 10 feet, all expsed piping shuld be stenciled "Feed Line t Backflw Preventer DO NOT TAP" at 5 ft intervals. Drainage fr backflw preventin assemblies shall be prvided fr all installatins f DCV r RPZ t accmmdate discharge during testing r draining f the unit and fr RPZ relief valve discharges, as fllws: Fr RPZ devices, drainage capacity shall be sized t accmmdate bth intermittent discharges and a catastrphic failure f the relief valve. Refer t manufacturers flw curves t determine maximum discharge rate based n supply pressure r n-site pressure; whichever is greater. Discharge frm relief valves must be readily detectable t maintenance persnnel either visually r by means f water level alarms, flw indicatr lights, etc. All drainage frm RPZ's must be by gravity drains. Sump pumps are nt allwed unless they are sized t accmmdate the maximum discharge rate and cnnected t emergency pwer supplies. An air gap must be maintained between the RPZ relief valve pening and any discharge piping. The air gap must be at least twice the dimensin f the effective pening f the relief valve; but in n case less than 1 inch. Manufacturer's air gap fittings may be utilized prvided that they maintain a prper air gap and d nt enclse r cver the relief valve. These fittings are nly sized t handle intermittent and lw flw discharges. Additinal drainage capacity may be required t accmmdate a catastrphic relief valve failure.

3 IV. Discharge piping frm relief valves shall be terminated a minimum f ne inch abve any flr drain r ther receiving receptacle. Discharge piping cnnected t a strm sewer shall be equipped with backwater check valve. Discharge piping cnnected t a sanitary sewer shall be trapped and equipped with a backwater check valve. Discharge piping frm pits r ther structures must be terminated abve grade in an area nt subject t flding (generally ne ft abve the 100 year fld elevatin). The terminal end f the discharge piping must have a rdent screen and may need t be supprted by a headwall. Flap valves shuld als be cnsidered t prevent entry f cld air. All exterir drains shall be kept free f snw during winter. Pit Installatins Primarily due t cnsideratins fr access, safety and gravity drainage, it is preferred that backflw preventin devices nt be installed in pits. Where pit installatins are prpsed, hwever, they shall be designed: T be watertight with watertight manhles r access drs extending a minimum f 6 inches abve grade and lcated t allw natural light int the pit during testing/maintenance. With stairways, ladders r step irns. Fr crane access fr installing and remving large assemblies. With adequate hrizntal and vertical clearances t allw access t the device. With a full flw screened gravity drain terminating abve grade fr all RPZ installatins as detailed in the drainage requirements. With sump pumps r gravity daylight drains fr all DCVA installatins. With flrs pitched t drain. With adequate grund cver t prevent freezing. With surface grading t divert runff away frm the entrance way. Semi-buried pits fr berm installatins may be necessary t satisfy gravity drainage requirements. V. Abve Grade Installatins- Prtective Enclsures An abve grade installatin is generally necessary t prvide gravity drainage frm RPZ devices. The additinal benefits f imprved access and enhanced safety are als realized with an abve grade installatin. Tw cmpanies, "Ht Bx" and "Hydrcwl", have designed prefabricated insulated enclsures that prvide heat, gravity drainage and remvable access panels fr servicing and testing. As an alternate, wd frame, fiberglass, steel, masnry r precast cncrete structures may be utilized. All enclsures shall be designed: VI. With a flr elevatin that is at least 6 inches abve finished grade. T prvide adequate clearances arund the device t access the test ccks, shutff valves, check valves and relief valve. With electric heaters r heat trace wire fr any water service used year rund. With prvisins fr natural r artificial light. With full flw gravity drains accrding t the drainage requirements. With security measures such as lcking drs and panels, flw alarms r flw indictr lights, pwer indicatr lights, etc. Installatin Within a Building

4 Where cntainment at the prperty line cannt be achieved r is waived based n extenuating circumstances, installatin within a building is ften desirable as the unit can be installed in a mechanical rm r ther area that has heat and light. Access and drainage cnsideratins must als be satisfied and the devices shuld be lcated t avid electrical panels, areas f excessive heat, etc. 1. Abve grade installatins shall be prvided with adequate clearances and discharge can be directed t flr r drains r thrugh a sidewall abve grade via screened luvers, scuppers, pipe sleeves with flap valves, etc., in accrdance with the drainage requirements. 2. Belw grade r basement installatins are acceptable fr DCVA's. RPZ's are nly allwed belw grade where ne r mre f the fllwing cnditins can be met: Where an adequate gravity drainage system is prvided t accmmdate a relief valve failure. Where water level alarms are installed t detect flw frm the device and alert maintenance r security persnnel. Where sump pumps are sized t accmmdate a relief valve failure and are cnnected t emergency pwer. Where the flr area and vlume belw the device culd accmmdate discharge frm a relief valve failure. Fr 2 inch and smaller units, 2,000 cubic feet is generally acceptable. Fr larger units, the time t submerge the device based n the maximum discharge rate and flr area/vlume shuld be n less than 8 hurs. In any f the abve cases, the prperty wner must be made aware f the ptential fr water damage in the event f a discharge. VII. Submissin and Apprval f Plans In accrdance with Sectin 10 f the Crss Cnnectin Cntrl manual, the submissin f plans and specificatins fr the installatin f backflw preventin assemblies must include the fllwing: VIII. 0. A site plan (t scale r with dimensins) f the facility cntaining a general lcatin map, name and address f facility, prperty lines, buildings, the size and lcatin f public water main(s) and all fire and dmestic water services, meter pits, yard piping and hydrants, pumper cnnectin(s), intercnnectins, and the lcatin f the prpsed backflw preventer(s). 1. A plumbing flr plan (plan view) r partial flr plan indicating water services, name and address f facility, water meter layut, prpsed backflw preventer(s), bster pump system, flr drain(s) and all nearby bjects (examples: electrical panels, bilers, chillers, strage tanks, fire pumps, fire sprinkler risers, etc.). The plan must be drawn t scale r with dimensins indicated frm walls and all nearby bjects. 2. A vertical crss sectin(s) f the prpsed installatin with elevatins frm flr, ceiling, utside grade and all nearby bjects. 3. All drawings must include the name and address f the facility, be stamped and signed by the designer and have a clear space fr apprval stamps. Engineer's Reprt An engineering reprt must be included with the plan submittal. The reprt must describe the prject in detail. Items that shuld be included r described in the reprt include:

5 0. General use f water within the facility; 1. Size and descriptin f all fire and dmestic water services; 2. Number f flrs within the facility 3. Actul r estimated maximum flw demand; 4. Pressures - existing and after the installatin f the backflw preventer; 5. Descriptin f the fire fighting system - indicate the A.W.W.A. Manual M-14 class f sprinkler service; 6. Descriptin f the prpsed installatin f the backflw preventer indicate the lcatin f backflw preventer, drainage, lighting, heating, access t unit, square ftage f the flr level where the backflw preventer is t be lcated; 7. Descriptin f the existing r prpsed bster pump system, answering the fllwing questins: A. After the installatin f the prpsed backflw preventer(s), will the Net Psitive Suctin Head (NPSH) required fr the prper peratin f the bster pump system be adequate? B. After the installatin f the backflw preventer(s) in the suctin line t the bster pump system, will the bster pump system perate prperly at peak demand t deliver adequate pressure t the highest elevatin and/r mst remte fixture unit r any ther peratin requiring a certain pressure? Nte: The New Yrk State Unifrm Fire Preventin and Building Cde Part 902.4c requires the minimum pressure at water utlets at all times t be as fllws: Fixture - nn flush valve - 8 psi Fixture - flush valve - 15 psi C. Des the bster pump system have a pressure cutff switch in the suctin line? What is the pressure setting f the switch? An existing r prpsed cutff switch must be set at the fllwing setting: Fr a cutff switch where the backflw preventer is lcated upstream f the bster pump(s) - set at 10 psi. Fr a cutff switch where the backflw preventer is lcated dwnstream f the bster pump(s) - set at 20 psi. IX. 8. The need fr dual backflw preventers. Des the facility need a cntinuus water supply? 9. The elevatin and lcatin f the 100 year fld plain in relatin t the facility. A reduced pressure zne (RPZ) backflw preventer must generally be installed 1 ft abve the 100 year fld plain elevatin. 10. An inventry f any existing cntainment devices t include the make, mdel, size and serial number f the device. Current annual test reprts must als be submitted. The degree f hazard fr these services must be determined t insure that the device prvides the crrect prtectin. Certified Testing and Cmpleted Wrks Apprval After an apprval f plans has been issued and the assembly has been installed, it must be tested by a certified tester. The designer (r water supplier) is then respnsible t certify that the installatin was dne in accrdance with apprved plans; r describe any changes r submit "As Built" plans as apprpriate.

6 The initial test results and certificatin are then submitted t the water supplier and apprving agent fr issuance f a Cmpleted Wrks Apprval. DOH - Frm 1013 has been designed fr bth the certified test results and the designer's certificatin f the installatin. After issuance f the Cmpleted Wrks Apprval, the assembly must be tested at least annually by a certified tester with the results reprted t the water supplier.