5.3.5. Valves

SAS4A/SASSYS‑1 contains a simple valve treatment in which the user can specify the valve pressure loss coefficient as a function of time. The pressure drop, \(\Delta p_{\text{v}}\), through a valve is calculated as

(5.3-180)\[ \Delta p_{\text{v}}\left( t \right) = G_{2}\left( t \right) \frac{w \left| w \right|}{2\rho A^{2}}\]

where

\(w\) = coolant flow rate through the valve

\(\rho\) = coolant density

\(A\) = nominal valve flow area

\(G_2\) = valve coefficient

The user supplies a table of \(G_2\) as a function of time. The table is used to obtain an average value of \(G_2\) for each step of the transient. With this treatment, a valve is never entirely shut; although \(G_2\) can be made very large so that the flow through the valve becomes very small.

Some care must be taken in setting the input for a valve. The table of \(G_2\) vs. \(t\) goes in the DTMPTB and TMPMTB input arrays, but in addition \(G_2 \left( t=0 \right)\) goes in the input variable G2PRDR (ELL) for the element IELL corresponding to the valve. During the steady-state initialization, G2PRDR may be adjusted by the code if necessary to achieve a steady-state pressure balance. At the end of the steady-state initialization, if G2PRDR does not match the table entry for \(t=0\), then a constant value is added to all of the entries in the DTMPTB table for the valve so that the table does match G2PRDR at \(t=0\). Up to eight valves can be used. Each valve can use its own separate valve coefficient table. It is also possible to use the same table for two or more valves but this option must be used with care. All valves using the same table should have the same steady-state valve coefficients and the same time dependence. Also, if a valve table is adjusted to achieve a steady-state pressure balance for one valve, the adjustment may not be appropriate for other valves using the same table.