1999 TECHNICAL INQUIRY ASME PTC 6A-1982 Section VI In order to conduct performance tests on two 50MW PC units, what

is the intent of Section VI of PTC 6A, and confirm the following: Question 1 Refer to Section VI-7. In calculating for feedwater flow, in this application, we are using heat and material balance around the deaerator. All incoming flows to the deaerator are measured with the exception of the extraction steam and HP heater drains. In this calculation we have considered deaerator level (change in storage) as an indication of unaccounted flows (losses) in the cycle. However, in calculating for throttle flow (Section VI-9), where the above calculated feedwater flow is already being used in the equation, we feel that it would not be the intent to consider deaerator level changes again for the unaccounted changes in storage. Answer 1 The deaerator level change must be included in both the deaerator mass and energy balance and in the calculation of unaccounted-for leakage. The correct calculation of the mass and energy balance around the deaerator requires the inclusion of the deaerator level change (change in storage) in this calculation. The dearator level change must also be included in the calculation of the unaccounted-for leakage since this is a calculation of the overall mass balance around the entire cycle. For the calculation of unaccounted-for leakages see PTC 6-1996, para. 3.5.3; for the calculation of test throttle flow see para. 5.4.1. Please be careful of the sign convention used in both calculations. Note that inclusion of a rise in deaerator level will result in a smaller calculated feedwater flow and a smaller calculated unaccounted-for leakage. Question 2 Refer to Section VI-9. In conjunction with calculating for throttle flow in question 1, we also feel that it would not be the intent to consider condenser hotwell level (changes in storage) as part of unaccounted flows, since condensate flow to the deaerator in our application is actually measured and that any resulting changes in unmeasured flows would be reflected in the deaerator level (which directly affects the calculated feedwater flow). Answer 2 The hotwell level change must be included in the unaccounted-for leakage calculation since this is also part of the overall mass balance around the entire cycle. We agree that the measured condensate flow includes the measurement of condensed steam exhausting form the turbine and the hotwell level change. However, for the proper determination of the unaccounted-for leakages, water storage in the condenser, deaerating and extraction feedwater heaters, steam generator drum(s), and other storage points within the cycle are to be taken into account.

THE FOLLOWING IS A FOLLOW-UP OF THE ABOVE INQUIRY In your response No. 1, you're not implying that we compensate for deaerator level twice? I mean, when we calculate for feedwater flow using mass balance around the deaerator we are compensating (+ or -), depending on the deaerator level change, i.e., Feedwater Flow = [all incoming flows to deaerator (+ or - ) flow due to deaerator level change]. When we calculate for steam flow using mass balance around the entire cycle, the steam flow equation we are using is Steam Flow = [feedwater flow + superheater attemperation flow - unaccounted for change in storage - etc....,]. In this equation, [all incoming flows to the deaerator (+ or -) flow to deaerator level change] will be substituted for feedwater flow and if we consider deaerator level change again for unaccounted flow for change in storage, the equation will become: Steam Flow = [all incoming flows to the deaerator (+ or -) flow due to deaerator level change + superheat attemperation flow - [(+ or -) flow due to deaerator level change (+ or -) flow due to condenser level change, etc....]. Depending on the sign used, the deaerator level compensation can be either doubled or cancelled out. In your response No. 2, how much of a level change in the extraction feedwater heaters and steam drum would justify including these compensations in the unaccounted flow? ANSWER Your current inquiry, and its predecessor, bear only a tenuous relation to PTC 6; your questions are concerned principally with the arithmetic of tracking flows in a steam cycle. The Committee's scope does not include answering questions of this type and it is unlikely to provide answers to similar questions in the future. The Committee will address your query regarding the compensation of deaerator level change in the flow calculations using the illustration in the accompanying example. This example is for a typical steam cycle with a high-pressure feedwater heater (HP #5) downstream of the deaerator, the latter being designated as feedwater heater #4. For simplicity, the example assumes that the drains flow from the HP heater to the deaerator is measured. For some plants, the quantity is calculated from the mass and energy balances for the appropriate HP heater(s). It is also assumed that the superheater and reheater attemperator spray flows are taken from a point downstream of the boiler feed pumps but upstream of the HP heater. The variations among steam cycles in the configurations for the HP heater and attemperator spray flows will not change the essential features of this illustration. In the example, a drop in the tank level is signified as a positive quantity. Likewise, the measured external leakages are positive quantities. With these conventions, the term unaccounted losses in Equation 3 will be positive if the total drop in inventory exceeds the summation of the measured leakage flows, i.e., there is a net out-leakage flow which is not accounted for by the measured leakage flows. Note that Equation 3 assumes that all unaccounted losses take place in the boiler.

It may be seen that the term for the change in the inventory of the deaerator (represented by change in deaerator level) is used in both the mass and energy balances for the deaerator (Equations 1 and 2). This term, together with those for the inventory changes in the condensate tank/condenser hotwell and steam drum, also appear in Equation 3 for the determination of steam flow. In the example, equations 1 and 2 are used to solve for the term QFOH4 directly to optimize the calculation algorithm. Alternatively, the quantity QEIH4 may be determined instead and substituted into Equation 1 to solve for QFOH4, using the term QDEA together with other measured quantities. In either of the two cases, the term QDEA would appear to be cancelled out by combining Equations 1 and 3, as pointed out in your second enquiry. However, the effect of the term QDEA has been embedded in Equation 2 used in the determination of the term QFOH4 or QEIH4. Your last question, regarding the acceptable magnitude for the unaccounted flow, can be answered only in terms of the acceptable magnitude in the determination of the throttle flow and the test result. To satisfy PTC 6, par. 3.5.3 limits the unaccounted loss to 0.1% of the full-load throttle flow.

See below for a typical simplified steam cycle, calculations and flow diagram. Also, please note that
"ASME procedures provide for reconsideration of this interpretation when or if additional information is available which the inquirer believes might affect the interpretation. Further, persons aggrieved by an interpretation may appeal to the cognizant ASME committee or subcommittee. As stated in the foreword of our standards, ASME does not "approve", "certify", "rate", or "endorse" any item, construction, proprietary device or activity."

TYPICAL SIMPLIFIED STEAM CYCLE FEEDWATER AND STEAM FLOW CALCULATION PROCEDURES The condensate flow from the deaerator is calculated by mass and energy balances around the deaerator. From mass balance QFOH4 = QDOH5 + QFIH4 + QEIH4+ QDEA QVENT From energy balance QEIH4 = [ QFOH4 x HFOH4 + QVENT x HVENT QFIH4 x HFIH4 QDOH5 x HDOH5 QDEA x HDEA ] / HEIH4 Substituting (2) into (1) and solving for QFOH4 gives QFOH4 = [ QDOH5 (HEIH4 HDOH5) + QFIH4 (HEIH4 HFIH4) + QDEA (HEIH4 HDEA) + QVENT (HVENT HEIH4) ] eq. 1

eq. 2

/ (HEIH4 HFOH4)

The feedwater flow to and from the HP heater is given by the mass balance around the envelope comprising the boiler feed pumps and the take-off points for the attemperator spray flows QFOH5 = QFOH4 + QGIP QGOP QDSUP QDRH The main steam flow from the boiler is given by QMOB = QFOH5 + QDSUP UNACCOUNTED LOSSES = QFOH4 + QGIP QGOP QDRH UNACCOUNTED LOSSES eq. 3 where UNACCOUNTED LOSSES = (reduction in inventories in condensate tank / condenser hotwell, deaerator storage tank and boiler drum) measured condensate and feed system losses = (QCOND + QDEA + QBLR) ( measured external losses)

Notation for the terms used in the above equations are as follows: Flow Quantities QDOH5 QDRH QDSUP QEIH4 QFIH4 QFIH5 QFOH4 Drains flow from HP Heater #5 Reheater attemperator spray flow Superheater attemperator spray flow Extraction steam flow to deaerator (heater #4) Condensate flow to deaerator Feedwater flow to HP Heater #5 Condensate flow from deaerator

QFOH5 QGIP QGOP QMOB QRIC QVENT

Feedwater flow from HP Heater to boiler Gland seal flow into boiler feed pumps Gland seal flow from boiler feed pumps Main steam from boiler Recirculation flow from boiler feed pumps to deaerator (= 0) Vent flow from deaerator ( = 0)

Change in Inventories QBLR QCOND QDEA Enthalpies HDEA HDOH5 HEIH4 HEIH5 HFIH4 HFOH4 HVENT Enthalpy of water in deaerator storage tank Enthalpy of drains from HP Heater #5 Enthalpy of extraction steam to deaerator Enthalpy of extraction steam to HP Heater #5 Enthalpy of condensate to deaerator Enthalpy of condensate from deaerator Enthalpy of vent steam from deaerator Drop in inventory in steam drum Drop in inventory in condensate tank and condenser hotwell Drop in inventory in deaerator storage tank