Page 17 - Gi flipbook May 2018

This article will review opportunities
for improving temperature control of FIGURE 1: Water bath heater
indirect heaters used in preheating
applications on gas distribution networks.
As gas is transferred from high
pressure transmission networks to
lower pressure distribution systems,
its temperature drops due to the
thermodynamic principle known as
the Joule-Thompson effect. Safe
operation of distribution networks is
ensured by preheating process gas,
such that its temperature is above
freezing following the reduction in
pressure. Indirect heaters used for
natural gas preheating form part of
critical network infrastructure and
their reliability is required to ensure
safe delivery of gas to end users. FIGURE 2: Heat loss comparison FIGURE 3: Daily gas flow fluctuations for a
The first UK preheating assets PRS site
installed in the 1970s were water bath
heaters (WBHs). These were
conceived during an era of energy
abundance, and featured a
characteristically simple design
selected for its high reliability and
long asset life.
Innovation driven by the growing
importance of energy conservation
resulted in the gas industry’s adoption
of modular condensing boiler houses inability of conventional indirect safe operating temperature.
(BHs), which achieve higher thermal heating technology to maintain a Currently, the most practical
efficiency by reducing combustion steady outlet temperature, particularly solution for gas networks to avoid
losses and extracting heat from when embedded in a dynamic system underheating is to increase the target
exhaust gases. There are over 400 such as a gas network, where gas flows setpoints on heating systems. A higher
WBHs and 500 BHs currently in use and station operating pressures see setpoint provides a larger energy
on UK gas networks. large inter-hour swings, creating buffer within the system, but also
The existing consensus around significant load diversity. leads to a greater proportion of
water-based (single-phase) indirect As part of a strategic pipeline heat energy being used ineffectively,
heaters was established as they study performed by ProHeat and SGN, contributing to over-delivery of heat
represent a cost-efficient means of the overheating loss was evaluated beyond what is required. Thus, low
heating natural gas. However, there across multiple pressure reduction temperature events caused by rapid
remains an unanswered question stations (PRSs). Problems associated increases in loads are often mitigated
concerning what portion of the heat with poor temperature control can be at the expense of energy conservation.
supplied to gas may be considered divided into two categories: Preheat losses account for heat not
useful, in other words heat that underheating and overheating, delivered to gas, escaping to the
contributes to maintaining an ideal outlet respectively referring to the environment either through flue-gases
gas temperature to prevent freezing. fluctuation of station outlet or from hot surfaces. As shown, the
This doubt arises from the inherent temperature below and above ideal BH’s improved thermal efficiency
results in a significant reduction in
heating losses. Nevertheless,
As gas is transferred from high overheating losses for both cases
represent 16.7 per cent and 20.8 per
pressure transmission networks to cent of the heater’s fuel consumption.
lower pressure distribution systems, Improving temperature control has
the benefit of reducing overheating
its temperature drops... safe operation losses, whilst also ensuring network
resilience with reduced underheating.
of distribution networks is ensured by Temperature control can be
preheating process gas, such that its improved with a range of techniques
that do not require asset improvement.
temperature is above freezing following Perhaps the most obvious
the reduction in pressure improvement involves upgrading the
control system itself. Preheating

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