Blending stations with prescribed quality

There are stations in the network where two or more gases come together and where at the outlet a gas with a prescribed quality is required. Such a station consist of flow controlled compressors and reducers. There is a specific ratio of the flows. This ratio is determined by the prescribed quality and the qualities of the entering gases. In dynamic real time the quality of the entering gases is not chancing instantaneously if the ratio is changed. In a stationary simulation this is different. Suppose the flow of one inlet of the blending station is coming from different supplies with different qualities. Chancing the flow at an inlet may influence the ratio of the supplies upstream, and as a result the inlet quality changes. You can imagine a case where if more gas is wanted at an inlet, that this more gas has a higher heating value, whereas if less gas is wanted the gas has a lower heating value. This situation can give rise to an unstable simulation or eventually divergence. The solution lies in a good linearisation of the blending equation. This will be presented here.

Influencing supply flows by pressure drop.

We had a simulation of our network with high and low calorific gases. All pressures and qualities were within bounds. The demands are energy demands, which means that demands are in terms of energy per time rather then volume per time. The simulation was an optimization and high calorific gas was expensive whereas low calorific gas was cheap. We enlarged the maximum allowable flow of a low calorific supply. Because low calorific gas is cheap, more of this was going through the system, but because more flow is required now, a pressure went below its minimal value. We found a way to influence the supply flows by the pressure drop. Again the solution lies in the linearisation. This will be presented here.

1 Introduction

Gasunie is a Dutch company which buys gas, transports gas and sells gas to energy distribution companies, power stations and large industrial users. Holland is a small country of approximately 150 miles x 200 miles. We have a high pressure gas net of 3000 miles with pressures ranging from 600 to 1000 psi with supplies gas to a regional net with pressures ranging from 100 to 600 psi. Gasunie has the policy of buying gas of different qualities. This is different from other companies who require the gas at the supplies to be at some standard quality. However with our customers we do have contracts with quality constraints. Because of this the high pressure network is subdivided in parts with different quality. The dominant gas in our system consist mainly of 86% methane and 14% nitrogen, so the heating value is medium. Most of our inland sales require this medium quality gas. A number of offshore supplies have a high quality.

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