Stress Analysis in District Cooling Projects

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Caesar II Application in the District Cooling

IndustryBy Amer Abdul Aziz

Table of content• About DC PRO Engineering• What is District Cooling• Why District Cooling• Piping Stress Analysis in the DC Domain• Sample Project• How Caesar II and FEATools Help Us in Our Projects• Improvement Suggestions

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About DC PRO Engineering


• DC PRO Engineering is a recognized world’s leading authority in the field of District Energy and renowned regional sustainability leader in the field of Green Building MEP Designs.

• DC PRO Engineering was established in Sharjah in 2006 and now has offices in Sharjah, Dubai, Abu Dhabi, Riyadh, Montreal, Washington D.C., and soon in Australia.


• DC PRO Engineering core services include design consultancy and project management for district energy and green building MEP projects.

more than 3 million tons of refrigeration (equivalent to

approximately 11 million kW)

more than 70 million sq. ft of Green Building MEP projects


• DC PRO Engineering is the winner of Climate Control Award for Best District Cooling Consultant in the GCC for the last 6 years in a row.

• DC PRO Engineering was ranked 32nd from the top 100 SMEs in Dubai out of the 4,500 nominations for the year 2015.

• DC PRO Engineering was awarded Sustainable Project of the Year twice by The Research Council for the year of 2014 for Muscat Innovation Park project and in 2011 for Al-Bustan project in KSA.

• DC PRO Engineering has Dubai Chamber Corporate Sustainability and Responsibility (CSR) label.

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What is District Cooling

What Is District CoolingDistrict Cooling Plant

ETS Room

Thermal Storage Tank

BTU Meter

Piping Network

DCP Different Architectural Structures

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District Cooling Major Equipment Components

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Centrifugal Chillers Controls & BMS Water Pumps

Inside The Plant Room

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Cooling Towers Water Piping NetworkThermal Storage Tank

Outside The Plant Room


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Energy Transfer Station Energy Meter Fan Coil Unit

At The Consumer Side


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Why District Cooling

Top View of A Building with Air Cooled Systems

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Air Cooled DX Air Cooled Chillers

Top View of A Building with District Cooling System

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Water Cooled ChillersDistrict Cooling

Why District EnergyBased weather conditions in Abu Dhabi

• Typical Air Cooled Chiller Plant Demand: 1.70 KW/TR

• Typical District Cooling Plant Demand: 0.85 KW/TR

• Typical District Cooling Plant with Thermal Energy Storage System Demand: 0. 68 KW/TR

Therefore, District Cooling can reduce the air conditioning power demand compared to conventional air cooled chillers by 50%; further reduction by 60% can be achieved when Thermal Energy Storage is used which will also lead to reduction in infrastructure and distribution costs.

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Stress Analysis in District Cooling Systems

Stress Analysis in District Cooling Systems• All the components of a DCS (plant, distribution network,

and consumer buildings) include piping that require stress analysis.

Location Pipe Material Applicable Design Code

PlantCS (chilled water pipes) ASME B31.1

CS or GRP (condenser pipes)

ASME B31.1 (CS)ISO 14692 (GRP)

Distribution Network CS ASME B31.1 ASME B31.4

Building Side CS ASME B31.9

Stress Analysis in District Cooling Systems• The main challenge in the stress analysis of DCS is the huge

temperature differences between the ambient installation temperature and the fluid operating temperature, which result in substantial thermal expansion loads.

Typical Temperatures in a DC Project in Dubai

Ambient Temperature (oC) Fluid Temperature (oC)

Summer Winter Supply Return50 10 4.4 13.3

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Example Project – An Energy Center in Oman

Example Project • Project: Energy Center in Oman:

– Size: 17,500 TR

– Description: The energy center consists of:• 1st floor where chillers and cooling towers are located.• Ground Floor where the primary, secondary, and condenser

pumps are located.• Basement Floor: Heat Exchangers.

– Scope: Several failures occurred in the field during initial filling, testing and commissioning. Contractor appointed DC Pro Engineering to conduct a detailed stress and structural analysis and propose technical solutions to resolve all failures.

Example Project• Support System Description:

– The support system was all built from the columns; no supports from the ceiling or the ground

– The GRP pipes were supported every 3 m.– The steel pipes were supported every 9 m.– All the pipe shoes were bolted to the beams.– The stress analysis for the chilled water and condenser water systems were conducted

by the respective suppliers.

THE FAILING PART

Example Project

Actual Image of the failing Support

Example Project

Actual Image of the failing Support

Example Project

Sheared Anchor Bolts on the Column Base Plate

DC PRO Engineering Solution

1200 mm diameter expansion joint failed as well due to improperly sized control bars

DC PRO Engineering Solution• The first step was understanding the causes of the failure:

– The support system was all built from the columns, causing shear stress on the side base plates.– The failing supports carried two Φ1600mm Condenser GRP Pipes and one Φ1200mm chilled water

pipe.– The stress analysis of each piping system was conducted by the respective supplier independently.

The support loads were never coordinated between the two services.– Consequently, supporting structures and bolts were not designed based on actual combined loads

obtained– The GRP system stress analysis was conducted using the allowables of ASME B31.3!– The pipes were heavily clamped, with the shoes bolted to the beams.

THE FAILING PART

DC PRO Engineering Solution- DC PRO Engineering conducted full stress analysis for both the chilled water and condenser water systems

using Caesar II 2016 to obtain resulting load combinations at all support locations

- FEATools was used to calculate the Tee SIF’s for the chilled water piping system.

- Using stress and structural calculations, it was evident that supports and all anchor bolts were undersized leading to the failure in piping system

- All bolted pipe shoes were replaced by resting/guided shoes with a PTFE plate to reduce the pipe friction and relieve the pipe stresses due to thermal expansion.

DC PRO Engineering Solution

DC PRO Engineering Solution– The supporting system was re-designed using the loads retrieved from Caesar.

– All base plates and anchor bolts for the supporting structures were redesigned based on actual resultant loads

– There was no possibility to install supports from the ceiling or ground. There was no possibility of changing the location of the base plates.

Re-designed Base Plate + Corbel The failed support part Re-Constructed

Ex-1 DC PRO Engineering Solution• Following DC Pro’s design and modifications, the system was

commissioned and operated successfully on September 25th, 2016.

• Due to the failure that occurred initially, remodeling and redesign process carried out by DC PRO Engineering was the top priority for all stakeholders involved in the project.

• Caesar II helped in answering all doubts & concerns raised by the client related to the support structure. Detailed modeling techniques and level demonstrated by DC PRO Engineering gave the client the confidence and assurance even prior to re-testing of the piping system.

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How Caesar And FEATools Improved Our Designs

How Caesar II and FEATools Help us in Our Projects

• Caesar II allows optimization of the support system where no extra supports are added, and supports are not oversized due to common practice or inaccurate assessment of loads.

• Caesar II helps us answering customer doubts concerning the pipe system design. For example, we are always faced with the question: Why are you not installing thrust blocks on elbows and end caps on your piping?

• We always prove by Caesar II models that for underground piping, all pipe stresses are well below the code allowable, and that the elbow loads are adequate.

• For above-ground piping, we always prove that anchor blocks on the elbows and end caps usually lead to pipe failures or abnormally high loads on the thrust block. Pipe flexibility is the always the answer to handle the expansion loads in district energy piping.

How Caesar II and FEATools Help us in Our Projects

• FEATools helped us a lot by giving us the correct SIF values for the Tees so that the Tees are not modeled as rigid bodies anymore. In the “pre-FEATools” time, we always had failures in the thermal expansion load cases; in those times we had to use expansion joints or expansion loops to resolve the failures. Now we know that was unnecessary in most of the cases!

• FEATools increases the accuracy of the model and saves modeling time consumed in calculating the SIF’s.

Example of The Effect of FEATools

• This is an actual chilled water piping design for a project in Qatar.

– Before FEATools is applied the system is failing the expansion load case with a stress ratio 133% at node 1490 (not surprising this is a TEE!)

– The same model is run with FEATools and now the maximum stress ratio is 57% at same node 1490.

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Improvements We Would Like To See

Improvement Suggestions• We suggest the following improvements to be made in Caesar II to enhance

the modelling experience:– Caesar II should be able to read IFC files from BIM Software. Lately BIM is a

requirement in every project, and it is time consuming to model the piping twice on Revit then on Caesar II.

– Isogen configuration tool has to be improved. It is very hard to adjust the properties and layout of the isometric drawings.

– Caesar II should add “Bulk modification” tool for elements using the database. For example, sometimes we need to change the properties of the expansion joint, which has to be done for each item alone.

Improvement Suggestions• We suggest the following improvements

to be made in Caesar II to enhance the modelling experience:– HDPE pipes to be added to the database.

HDPE pipes are standardized pipes, not like GRP. Right now we have to enter as a user defined material and use the alpha-tolerance in the temperature fields to get the expansion coefficients.

Improvement Suggestions• We suggest the following improvements

to be made in Caesar II to enhance the modelling experience:– It would be nice that Caesar II provide

the possibility to add a user defined support tag number, and to be able to sort the loads as per this support tag in the reports.

– Right now, Caesar shows only the directional restraint of the support. A resting support could be one of several types with the same directional restraint as the following images show.

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Thank You!Any Questions?