CAPEX & OPEX of tunnels vs. direct buried systems

[Len-IDEA]

Posted on behalf of an IDEA member organization:

As we do long term planning, one of the areas we always explore involves the options for expansion of our steam system due to projected load growth.  While tunnel installations bring the benefits of access to all piping, insulation, anchors, etc., the upfront and O&M costs are major considerations.  The alternative – direct buried piping and vaults – provides the access to valves, traps and expansion devices needed to operate the system, but leaves the bulk of the piping and insulation only accessible via excavation. 

We would be interested in hearing from members with any data comparing their experience (particularly capital and O&M costs and service life) for tunnels and direct buried systems.  As this may be difficult to quantify, any anecdotal input would also be welcome.  Thanks!

[dww]

Tunnels are, of course, the gold standard for distribution systems. Cost of construction is a major consideration even when you have very dense loads to serve.  There is nothing like being able to walk beside your piping and being able to touch it.  I have designed replacement condensate lines and pipe supports in one client's 100 year old tunnel system for what is the 4th or 5th time.  The steam pipes are still going strong.  Shallow culverts and direct buried conduit or bonded systems may also give cost effective service, but are very dependent on construction quality in my experience.  If you are doing long range planning for  system expansion I strongly recommend looking at hot water and the direct buried piping systems being used in Europe.  They are much more efficient than steam and can include direct buried pipe and valves which can be monitored by a leak detection system capable of locating a problem area within a few feet.

David Wade P.E.

RDA Engineering Services

[Hugh Bahar]

Cornell’s steam distribution history:  https://energyandsustainability.fs.cornell.edu/util/heating/distribution.cfm

 

Our mantra is "keep it dry and it will last forever."

 

If you wish more historical information and lifecycle cost bases, click on the following link and catch up with Lanny, Frank Perry or Brian Wanck.  https://energyandsustainability.fs.cornell.edu/staff.cfm

 

Best,

Hugh

 

Hugh Bahar, PMP®

Project Manager / Sr. Engineer

Cornell University

Infrastructure, Properties and Planning

102 Humphreys Service Building

Ithaca, NY  14853-3701

Email: hr...@cornell.edu

Desk: 607-255-3853

Fax:  607-255-1968

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[David Christiansen]

I am working that issue as we speak - in recent history, installed piping has been direct bury - due to up front cost.  I believe that based on an assumed infrastructure of between 50 and 100 years, the life cycle cost for alternatives besides direct bury should be more attractive.  Don't expect to have anything to share for the next few months, but would be willing to share after we are done.

David Christiansen

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[Richard Francki]

At York University in Toronto, we have a split system.  The original steam/cw pipes are in tunnels.  Later on, when the ringmain was completed, a twinned inverted trench (inaccessible) with valve chambers was used to limit costs.

Two problems ensued:

Recurring (over 12 years commencing at startup) failures of expansion joints and supports eventually caused us to excavate the inverted trench to discover inadequate and failed steam pipe supports.

Chilled water leaks caused us to excavate (and chase along 400 ft) 12 yr old twin 20" cw pipes, to discover dramatically accelerated wastage from anaerobic bacteria flourishing in a cold and humid environment, principally under the hard calcium silicate support insulation at the brackets.  Huge costs and major disruption on campus streets.  Probably another 1000 ft of suspect pipe to be inspected, with some indications that more of the same will be found.

No such problems in the tunnel system.

Richard

Richard Francki

Assistant Vice-President 

Campus Services and Business Operations

York University

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[Ontiveros, Juan M]

Most of our system of nine miles of tunnels has been around since the 30's without significant issues for chilled water and steam. Good insulation and good water treatment is key but I believe our success has been the use of expansion loops rather than slip, articulated or bellows joints. Never have had a leak except from instrument nipples.

Our latest expansion is direct buried.  While tunnels were our standard this expansion is in an area without tunnels. Our subsoil is almost exclusively limestone so we no longer can afford tunnels. So are using hdpe pipe for chilled water and steel pipe for a hot water distribution system. So we are carefully employing expansion loops here also. For insulation we are using gilsulate. It is hydrophobic,  great isulating quality and provides electric insulation so no cathodic protection needed. 

We don't have good luck with preffab pipe due to workmanship issues.  The approach we used is conventional construction.  Dig trench, lay in pipe with expansion loops, test it, pour in gilsulate and backfill the soil. 

Juan Ontiveros 

Sent from my Sprint Samsung Galaxy S® 6.

[Riley, James G]

At Texas A&M, we have had great success installing underground piping systems using HDPE for both CHW and HHW with Gilsulate insulation for CHW piping 12 inch and under and all HHW piping.  HDPE has significant R-value, so we found that insulating CHW piping above 12 inches provided little additional benefit.  HDPE piping with 2499 resin is rated to 180 F at 100 psi, which meets all temperature and pressure requirements for most HHW distribution systems – but need to verify for the specific system.  We have found HDPE to be superior to steel or ductile iron piping for the following reasons:  eliminates all corrosion, better flow characteristics, greater flexibility (for bends and turns), easier installation, lower cost.

 

Regards,

 

Jim Riley  |  Executive Director

Utilities & Energy Services  |  Texas A&M University

1584 TAMU  |  College Station, TX 77843-1584

Tel. 979.845.1210  | jimr...@tamu.edu 

      http://utilities.tamu.edu

[Hugh Bahar]

Person-tunnels, if constructed correctly give optimum opportunity for inspection, addition of new in-tunnel utilities, and preservation of piping, guides, hangars anchors and insulation.  They tend to be first-cost prohibitive because of the quantity of construction materials and construction/excavation labor.  Densification of the underground utility environment in urban areas often limits the ability to install new person-tunnels, or person-tunnel laterals off of legacy tunnels.  The best person-tunnels that I’ve seen use existing building basements and/or corridors to reduce the direct buried distances.  Most of the benefits of person-tunnels without having to build them anew.

 

If direct-buried steel lines are used for steam, water-sealing of the insulation (i.e., pitwrap around foamglass) must be perfect.  If condensate is also direct buried in steel, it will tend to leak before the steam because of its aggressive nature and because most steam plant operations can’t or don’t adequately treat feedwater to ensure condensate return pH is optimal.  If condensate is buried adjacent to the steam line (usually the case), the heat from the condensate leak will rapidly deteriorate the pit wrap on the steam line and a few months after the condensate repair is complete, the steam line often fails from external corrosion.

 

Gilsulate and similar products have to be installed correctly, and if the condensate leaks adjacent to the steam line the trench will have to be vacuumed out with a vac-truck for repairs…a messy proposition as vac-trucks don’t tend to filter their tank exhaust to Gilsulate particulate size. 

 

Steam inside inverted tunnels on a grade beam isn’t a quick fix but it’s the best we’ve found for our environment.  Attention to detail is required regarding the waterproofing of joints, underdrain preparation, steam line slope and backfill selection.  Inside the steam vaults, expansion joints must be carefully designed and the joint vendor carefully selected (we’ve only found one vendor of sliding joints who makes them with high enough quality to ensure longevity) and the expansion joints maintained.  Large anchors are required inside the steam vaults, inaccessible in-tunnel steam line guides and slides must be specified and installed correctly (so steam lines don’t slip off and then get hung up and tear holes in the steam line), and the vaults themselves must be constructed to withstand anticipated loads from traffic, including emergency and construction vehicles as well as the rebar corroding de-icing salt environment found in many snow-belt locations.  If condensate is co-located with the steam in the tunnels, using properly specified stainless steel or schedule 80 steel is prudent for longevity.

 

Regarding steel chilled water distribution, pipe wrap, cathodic protection and carefully selected backfill are essential, as is maintenance of the chilled water cathodic protection systems via annual surveillance of each test station and monthly surveillance of rectifiers.  Isolation of the steel CHW system into manageable cathodic protection ‘zones’ via isolation joints has been essential for success as is an expert cathodic protection consultant.  As with condensate, it is essential to understand and maintain control of the chilled water chemistry itself.  For building laterals and smaller distribution mains HDPE is the direction we are migrating as it eliminates internal and external corrosion issues and the need for cathodic protection.  HDPE must be carefully selected, coupled, fused, backfilled, and traceable above ground. 

 

Most of the common failures/lessons learned that have been encountered in steam, condensate and chilled water systems have been encountered by Cornell, and our distribution designs have been and are still being thoughtfully improved.  If I remember correctly, we determined that for our environment direct buried foamglass and inverted tunnel on grade beam were similar life-cycle costs, and person-tunnels were much more expensive.  Two of the best individuals to contact regarding this subject are Frank Perry and Steve Little (Steve is retired from Cornell).  Frank and Steve will be able to recall our conclusions from our past life cycle cost analyses.  

 

Hugh

 

Hugh Bahar, PMP®

Project Manager / Sr. Engineer

Cornell University

Infrastructure, Properties and Planning

102 Humphreys Service Building

Ithaca, NY  14853-3701

Email: hr...@cornell.edu

Desk: 607-255-3853

Fax:  607-255-1968