8.8 FPS in 20" FM - Hold on!

[sjohns4]

The client I'm working for is in a tight spot. The short of it is if we dont heat up an existing 20" forcemain with temporary pumps there will be sewer on the ground and draining into tidal creeks to wash up on tourist beaches.

The velocity in the 20 year old DIP 20" forcemain will be about 8.8 FPS (8200 gpm) during peak conditions (hurricanes & tropical storms). We're near the cost and have really poor sandy soils and have no idea how well the fittings are restrained, although the line has held up fairly well to date. Peak pressure is only around 100 psi, so I'm more concerned about the extremely high velocities blowing fittings off. My role is as retainer engineer (not design engineer) for a municipality who doesnt own, but has interest in the forcemain. I'm wondering if I should recommend excivating around the 45's & 90's and adding extra blocking or other restraints? The line to date has only flowed at around 5000 GPM max.

Has anyone ever seen this sort of velocity in a line this large?

Any ideas?

 

[bimr]

Anchorage design should be based on pipeline pressures at least 25% greater than the maximum pump design shutoff head plus a waterhammer allowance with an appropriate factor of safety. The waterhammer allowance accounts for conditions such as pump power failure when you have an immediate shutoff of a pump.

THe use of flexible self-restraining pipe joints restrained with tie roads and clamps is recommended over thrust blocks, especially with poor soil conditions. One would think your soil conditions are poor considering the sand and high rainfall from a huricane.

You need to obtain the as-built drawings of the force main and ascertain the design pressure versus the proposed revised operating pressure.

Force main velocity is usually limited to 3-5 ft/sec to keep the headloss (and power costs) within reason. 7 ft/sec is probably the maximum reasonable velocity for municipal service for long term service.

The higher velocity that you have proposed is not unreasonable for short term service, but you are going to have evaluate whether the force main was designed to handle the higher pressure.

 

[dicksewerrat]

Where is this line? May have a few ideas. As BIMR stated you may have to dig up the bends. May have to encase some pipe sections in concrete

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com