Consolidation is inferring the behavioral description of a device by composing the behavioral descriptions of its components, e.g., deriving the qualitative differential equations (QDEs) of a device from those of its components. In previous work, Dormoy and Raiman described the qualitative resolution rule, which is a general rule for deriving QDEs of combinations of components. However, the qualitative resolution rule is intractable in general. As a step toward understanding tractable qualitative reasoning, I present a new QDE resolution rule, the qualitative difference resolution rule, that supports the tractable consolidation of components in which direction of flow is dependent on the signs of pressure differences. Pipes and containers are general types of components that match this rule. The pressure regulator example also matches this rule.