شرح كامل لمفهوم NPSH الخاص بالمضخات .. وكيفية الحفاظ على الطلمبات من التكهف 

هنتكلم النهاردة عن موضوع شائك في الطلمبات وبيحصل فيه لغبطة كتير، ومحل تساؤل دايماً في كل الانترفيوهات، وهو موضوع NPSH.
الأول ايه هو NPSH؟
بكل بساطة هو أقل ضغط الطلمبة محتاجاه عند فتحة السحب عشان تشتغل بدون ما يحصلها تكهف cavitation.
عشان نفهم موضوع NPSH بشكل أعمق، في كام نقطة عايزين نسجلهم سوا عشان هنحتاجهم قدام، وهم:
١- الطلمبات مصممة انها تتعامل مع المائع في حالته السائلة مش في حالة تبخير (غاز).
٢- نقطة impeller eye بيكون عندها أقل ضغط للسائل في الطلمبة، نتيجة سرعة دوران المروحة.
٣- درجة غليان المياه مش دايماً عند ١٠٠ درجة مئوية! الكلام ده بيكون عند الضغط الجوي (١ بار) بس، لكن لو الضغط قل درجة الغليان هتقل هي كمان، ودي اهم نقطة محتاج تفتكرها (لما الضغط يقل درجة غليان السائل هتقل).
السائل بيدخل الطلمبة بضغط معين، يعني مثلا هنا هتلاقي الضغط عند نقطة a بقيمة معينة، وبعدين انخفض لأقل ما يمكن عند نقطة b واللي هي impeller eye، وبعدين يرتفع تاني عند طرد الطلمبة (نقطة c).
الفرق بين النقطتين a و c هو الرفع الكلي total developed head.
فصول الفيديو:
- مفهوم ومعنى NPSH
- ما هو التكهف؟
- الفرق بين ضغط السحب الموجب المتاح والمطلوب
- طريقة حساب NPSH available
- طريقة اختبار NPSH للمضخات
- ١٠ طرق لمنع وتقليل التكهف في الطلمبات
This video will explain what is npsh. Why Net positive suction head is so important for pumps and how to determine the net positive suction head is also explained in this video.
As per hydraulic institute standard, npsh is the liquid energy, above the vapor pressure, at the pump inlet. Each pump requires some minimum absolute pressure at the pump inlet to avoid cavitation in the pump.
This minimum absolute pressure or absolute suction head in terms of the height of the liquid column, at the pump inlet, is known as net positive suction head. if the suction head at pump inlet drops below vapor pressure. The fluid will evaporate and generates small vapor bubbles. These bubbles carried along with the fluid, and collapse instantly when they get into areas of higher pressure. This will result in excessive noise and vibration, due to the collapse of vapor bubbles. it will damage the pump impeller and pump casing due to erosion. This whole phenomenon is known as cavitation. This minimum absolute suction head, at the pump inlet, which is above the vapor pressure represents the available net positive suction head.
To avoid cavitation, this absolute head at the pump inlet must be greater than vapor pressure. So if we subtract vapor pressure from this absolute inlet head. It must be always a positive term. And known as Net Positive Suction Head Available.
Till now, we have understood what is net positive suction head available. But how much net positive suction head required for a pump is calculated by testing a pump.
The most common method. Often known as the constant flow method is used to determine the npshr. In this method, the pump discharge valve is kept at the fixed open position. so that pump flow remains constant. However, the pump suction valve is throttled. Due to throttling of the suction valve, pump inlet pressure reduces. But, the pump developed head, and flow remains constant. Until a point where inlet pressure reduces to vapor pressure.
After each throttling of the suction valve, the Pump manufacturer notes down the Total head developed by the pump, and Net Positive Suction Head Available. The Pump manufacturer repeats this step, Until a point where the total developed head decreases by at least 3 percent. The net positive suction head corresponding to the 3% drop in pump developed head is known as NPSH3 or net positive suction head required.
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