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Use of contents on this sheet is subject to the limitations specified at the end of this document.P:\35000\135849 - SSWD Water Master Plan\Report Document\Master Plan Report\Final\Final SSWD Master Plan 080609.doc

WATER SYSTEM MASTER PLAN

14. SURGE ANALYSISPressure surges developed during pump startup and shutdown and under accident conditions such as loss ofpower to the pumps or inadvertent valve closure may exceed (steady-state) design values. Cavitation orexcessive pressure surging can lead to pipeline or component failure. Surge protection devices are oftenrequired to prevent the development of such conditions. This section provides a quantitative assessment ofthe Districts existing distribution system to identify potential surge issues.

14.1 MethodologyTransient pressures in water distribution systems are most common at pump stations, control valves, highelevation areas, and areas that are far from elevated storage (Wood, 2005). Specific events that can causesurge include:

1. Pump startup or shutdown (possibly due to power failure as described in Section 13)2. Rapid opening or closing of valves3. Rapid opening or closing of hydrants4. Pipeline break

The Districts hydraulic model was used to evaluate surge issues at the well sites that do not havehydropneumatic tanks. See Figure 14-1 for the location of existing surge protection within the system. Thisincludes the District hydropneumatic tanks, combined air valves, and air release valves throughout thesystem. The sudden shutdown of well pumps was analyzed at various locations and elevations throughoutthe water system. Because most of the well facilities have pump-to waste features, this analysis focused onsystem impacts due to pump shutdown. The proximity to elevated storage was not considered because theDistrict does not commonly utilize the elevated storage in the system. The Districts large transmission mainsall have combination vacuum/air release valves which can also help alleviate surge issues.

Surge issues for other situations such as opening or closing valves or hydrants were not modeled because theycan occur anywhere in the system and can be avoided by good operating practices such as opening andclosing valves and hydrants slowly. It is also not possible to model all of the potential events that could causesurge. Therefore, the surge analysis was focused on the occurrence of surge at a well pump and at boosterpumping facilities.

The analysis was done using the InfoSurge modeling software. This is a cost effective method of modelingthe system for surge potential because this software is fully compatible with the Districts hydraulic modelInfoWater software. A transient condition is created in the model by specifying an emergency shutdown or astartup at a pump. The model then calculates the pressure throughout the system thousands of times each

second.

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Water System Master Plan

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Existing Surge Protection

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TITLE

SITEPROJECT

DATE

0 3,000 6,0001,500

Fe

Legend

SSWD Well

Booster PumpStations (BPS)

Elevated Storage Ta

Ground Storage Tan

Pipes

Street

Highways

Capehart

McClellanNSA-1

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NSA-3

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N29

N31 N27

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Air Release Valve

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14.2 Analys is Resul t sThe facilities that were included in this analysis as representative facilities within the District are shown inTable 14-1 and 14-2. The high and low pressures resulting from abrupt pump shut down are also shown.

Table 14-1. Representative Facilities Analyzed for SurgeFacilit y VFD Location

Low/HighPressure

Results (psig)

Duration of negativepressure, seconds

N7 Rosebud No NSA 4/85 Less than 1

N29 Merrihill No NSAModel could notsolve

Less than 1

N33 Walerga No NSA -14/140 Less than 1 (near facility)

46Jonas/SierraMills

No SSA -14/142 Less than 1 (near facility)

71RiverDrive/Jacob

Yes SSA -2/83 Less than 1 (near facility)

--Enterprise/NorthropBPS

Yes SSA -13/132 ~ 2 (near facility)

A pressure surge analysis was also conducted at the Enterprise/Northrop booster pump station with pumpsA through E running fully open. Similar to the analysis at the wells, the minimum pressure was -13 psig at adead end near the booster pumps. The maximum pressure was 132 psi.

In the model analysis, negative pressures near the facility resulting from the booster pump shutdownoccurred for a duration of 2 seconds approximately 7 seconds after shutdown. This was longer than the

duration of negative pressures resulting from the well pump abrupt shutdown.

The surge analysis identified high and low transient pressures that could occur during an emergency pumpshutdown. The most extreme pressure range was found to occur immediately downstream of the pump andat the end of dead-end pipes near the pump, with the worst case occurring in the dead-end pipes. Pressuresurge within the looped distribution system did not appear to be a problem because the looping helps todissipate the pressure waves. Pressure surge at pump startup also did not cause the extreme pressures thatoccurred during pump shutdown. For example, Figure 14-2 shows the pressures for Well 46 pump startupand emergency shutdown at the end of a 6-inch dead-end line near the well. For this situation, the pressuresfor the pump startup did not vary much as compared to normal system pressures. However, the pressuresduring shutdown varied from 0 psi absolute (-14.7 psig) to pressures over 125 psig.

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-25

0

25

50

75

100

125

150

0 20 40 60 80 100

Time (seconds)

Pressure

(psi)

Pump Shutdown

Pump Startup

Figure 14-2. Pressures for Well 46 Pump Startup and Shutdown

14.3 Surge Protec t ion and Pressure Rel ie f Recomm endat ionsThis analysis was performed to give an overview of potential surge issues that could occur in the Districtswater system. Issues identified were limited to emergency pump shut down that would only occur if thepump was running during a sudden power failure event. Therefore, the risk of surge on the system is not

high. Below is a list of recommendations related to this surge protection analysis that should be consideredby the District.

1. Model parameters that were added to the well pumps to simulate the pump startup and shutdownwere selected based on typical parameters. A more detailed analysis would be necessary to calculatethe actual transient pressures at each well pump. This would include verifying the well pump setup(e.g. type of check valve installed at pump and the piping at the pump).

2. Because this analysis identified dead-end pipes as being vulnerable to negative pressures duringabrupt pump shutdown, it is recommended that a more detailed surge analysis be done at the wellsites that do not currently have surge protection. Based on the analysis, preventative measures couldbe identified such as looping the piping, two-way surge valve anticipators at the pump, orhydropneumatic tanks.

3. The surge analysis for the system indicated that, in general, there are limited concerns for surgeevents. However, some dead end pipes near pumps did show the potential for surge related lowpressures. A detailed analysis of every pipe in the system is beyond the scope of this report, butthese events are most likely to occur in areas that are farthest away from surge dampeningcomponents such as hydropneumatic tanks or elevated storage tanks floating on the system. Asshown on Figure 14-1, each of the subareas within the District contains well sites with surge reducing

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hydropnuematic tanks facilities. Areas that may have a higher potential for surge events, due to thelack of hydropnuematic tanks in the area, are the northern portion of NSA-1 and the northernportion of NSA-3. It is recommended that any detailed surge analysis of the system begin in theseareas with the highest surge potential. As previously noted, even these areas were found to havelimited potential for surge events.

InfoSurge allows the detailed modeling of surge protection devices. For example, a hydropneumatic tank wasadded to the Well 46 situation discussed above. Figure 14-3 compares the pressures for an emergencyshutdown before and after adding a 5,000 gallon hydropneumatic tank.

-25

0

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50

75

100

125

150

0 20 40 60 80 100

Time (seconds)

Pressure

(psi)

Pressure without Tank

Pressure with Tank

Figure 14-3. Pressures after Emergency Shutdown of Well 46 Pump With and Without Hydropneumatic Tank

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