Abstract
This study evaluates temperatures measured at district heating (DH) valves in manholes and their usability for non-destructively assessing the thermal performance of buried DH pipes. The study was conducted as a field test in which part of a DH network was shut down and the temperature decline in the valves was analysed in terms of absolute temperature and thermal response time from the DH pipe to the top of the valve. The calculated and measured supply pipe temperatures by the drainage valves were in good agreement, with 1% deviation. The valve measurement analysis from this study shows that the drainage valve has good potential to serve as a measurement point for assessing the thermal status of a DH network. However, the shutdown valve measurements were greatly affected by the manhole environment.
Highlights
A common rigid District heating (DH) pipe in a Swedish DH network consists of an inner service pipe of steel and an outer casing pipe of polyethylene (PE); between them is the PUR insulation, a gas-filled closed-cell structure
To fill this research gap, a “cooling method” has been developed for use during network maintenance with an excavated pit [5,6,7]. This cooling method is based on the temporary shutdown of part of the pipe network, while the cooling process is registered by thermocouples to capture the temperature decline in the service pipe
The purpose of this paper is to develop and evaluate a cooling method for assessing the thermal status of a DH network, focusing on an operating and unexcavated network
Summary
For measurements of pipes excavated during maintenance, previously investigated by the authors [8], four possible measurement positions and the importance of accurate measurements were highlighted as the key to using the cooling method with high precision. The measured value temperatures can be used for calculating the temperature decline of the DH pipe section. It is difficult to capture the actual absolute temperature in the supply pipe by means of valve measurements due to time lag of the valves and interactions with ambient air temperature in the manhole. The initial temperature of a supply pipe, before a shutdown, is important for estimating the absolute temperature decline of the supply pipe during the cooling period. Energy companies measure the supply temperature at several points in a DH network, mainly at the customers’ DH centrals and at the heating plant in the outgoing supply water, i.e., the supply temperature is known before shutting down part of the network for thermal performance assessment. The distance from the supply temperature measurement points to assessed parts of the network will be case specific, and the accuracy of these temperature measurements is governed by the point-to-point heat losses
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