Negative pressure C-900 CL150 sewer forcemain 1

[won13]

After surge analysis of forcemain (510gpm with C-900 CL 150 8-in pvc pipe), we found there will -16.5 ft negative pressure in case pump shuts down. Our analysis showed one 750 gallon surge tank can reduce to -2ft. But we don't want to install any surge tank.

What is commonly accepted negative pressure for PVC?
Is there any spec specifies allowable negative pressure for PVC?


Thank you.

 

[stanier]

You need to calculate the propensity to collapse due to buckling due to the external loads. This not only includes the external pressure (vacuum) but also live and dead loads. Live loads are from traffic, aircraft, railway and construction vehicles. Dead loads arise from soil loads, structures. In addition one needs to allow for the load from the water table above the crown of the pipe. The calculation involves the soil/pipe interaction. Then one has to calculate the combined loading.

The design process involves ensuring that your installation meets the material requirements for stress, strain, buckling, deflection and combined loading criteria.

There are AWWA standards that cover this matter as well as books on the subject of buried flexible pipelines.

In Australia we use AS 2566.1 Design of Buried Flexible Pipelines available from

http://www.standards.com.au.

The calculations are based upon the midified Iowa (Spangler) theory. For deep trenches the Marston theory can be used.

You dont say if the force main is potable water or sewage. If the former then there is reluctance to use air valves due to contamination concerns. However for sewage the Ventomat series of valves can be used. They have a unit specifically for sewage applications. Refer

http://www.ventomat.com.

 

[carletes]

Thanks Stanier,

My pipe is made of steel for service in a power plant in the main cooling water system.

Reviewing some information my main doubt now is the following: is there any criteria to decide if a pipe is flexible or rigid? I suppose that no all steel pipes are flexible and it will depend on its diameter, thickness and stiffness of surrounding soil. Does anybody knows anything about it? Because I have to design other steel buried pipes, some of them smaller, and I am not sure if all of them may be considered as flexible.

Best regards,

 

[stanier]

Carletes,

I would consider any steel pipe of Schedule 40 and greater to be rigid in this context.

The criteria used in design of a combined soil/pipe structure are stress, strain, deflection, buckling and resisatance to combined loading. All are able to be determined using the procedures in As 2566.1. You will also need to know the native soil modulus, dead and live loads, water table level.

Cooling water pipework in power plants is generally of large diameter and only moderate presssures. Also it is generally cement lined to prevent corrosion. The cement lining limits tyhe allowable deflection to 2-3% depending upon the manuafcturer and process used.

The other pespective is a risk assessment of your pipeline where you consider likelihood of an event and the consequences of failure. The latter in itself would normally constitutue such a high level for an asset such as a power plant that one would assess each and every line.

Its the clients call as to the level of engineering. Some folks may use this forum to provide a "get out of jail free card" so they can justify not doing the engineering. They need to think of what the Coroner might say if there is a failure!

 

[Chris73]

won13,

I have recently done a waterhammer study on a 150 mm PVC SDR 26 pipe and its critical negative pressure was -313 kPa. I would assume that a C900 8" is at least this. Good luck!

 

[plasguy]

The negtive pressure of 16 ft will be absolutely no problem wit 8" C900 PVC. Both structurally due to high collapse strength and for the gasketed joints which are tested to 25 ft negative pressure as part of ASTM and CSA standards.

For more info., contact IPEX at

http://www.ipexinc.com