Relief Valve Calculations And Equations - Control

دستگاه های کاهش فشار (PRD) به طور گسترده و مؤثری برای محافظت از تجهیزات فرآیندی مانند سیستم های لوله کشی، مخازن تحت فشار، ستون های تقطیر و سایر تجهیزات در برابر فشارهایی که بیش از میزان فشار طراحی شده بیش از مقدار از پیش تعیین شده ثابت هستند، استفاده می شوند. هدف از شیرهای فشار شکن جلوگیری از آسیب به تجهیزات، جلوگیری از آسیب به پرسنل و جلوگیری از خطرات احتمالی برای محیط زیست است.

Table of Contents

حداکثر طول خط دریچه تخلیه

The equation for the maximum length of a relief vent line is:

$$ L=\frac{9 \times P_1^2 \times D^5}{C^2}=\frac{9 \times P_2^2 \times D^5}{16 \times C^2} $$

پ₁ = 0.25 × [(تنظیم فشار × 1.1) + 14.7]

پ₂ = [(تنظیم فشار × 1.1) + 14.7]

L = حداکثر طول خط دریچه کمکی (پا)

D = قطر داخلی لوله (اینچ)

C = حداقل تخلیه هوا (پوند/دقیقه)

The first term in the equation, $\frac{9 \times P_1^2 \times D^5}{C^2}$, represents the pressure drop in the relief vent line due to friction. The second term in the equation, $\frac{9 \times P_2^2 \times D^5}{16 \times C^2}$, represents the pressure drop in the relief vent line due to the expansion of the gas as it flows through the pipe.

The equation is set equal to zero because it represents the maximum length of the relief vent line for which the pressure drop will not exceed the set pressure of the relief valve. If the length of the relief vent line is greater than the maximum length, then the pressure drop in the line will exceed the set pressure of the relief valve, and the valve will not open properly.

The equation can be used to design relief vent lines for a variety of applications, such as pressure vessels, boilers, and compressors. It is important to note that the equation is only valid for single-phase gas flow. If the fluid flowing through the relief vent line is a two-phase mixture of gas and liquid, then the equation will need to be modified.

Here is an example of how to use the equation to calculate the maximum length of a relief vent line:

The first step is to calculate the back pressure at the relief valve outlet:

P_1 = 0.25 * [(150 psig + 14.7 psia) * 1.1] + 14.7 psia = 42.6 psia

The next step is to calculate the inside diameter of the relief vent pipe:

D = 1.5 inches - 0.133 inches (wall thickness of Schedule 40 steel pipe) = 1.367 inches

Finally, we can substitute all of the known values into the equation to calculate the maximum length of the relief vent line:

L = 9 * 42.6^2 * 1.367^5 / 100^2 = 272 feet

Therefore, the maximum length of the relief vent line is 272 feet.

اندازه گیری شیر تسکین دهنده

شیرهای تسکین دهنده سیستم مایع و شیرهای کمکی به سبک فنری:

$$ A=\frac{G P M \times \sqrt{G}}{28.14 \times K_B \times K_V \times \sqrt{\Delta P}} $$

شیرهای کمکی سیستم مایع و شیرهای کمکی با کارکرد پایلوت:

$$ A=\frac{G P M \times \sqrt{G}}{36.81 \times K_V \times \sqrt{\Delta P}} $$

شیرهای تسکین سیستم بخار:

$$ A=\frac{W}{51.5 \times K \times P \times K_{S H} \times K_N \times K_B} $$

شیرهای تخلیه سیستم گاز و بخار (Lb./Hr.):

$$ A=\frac{W \times \sqrt{T Z}}{C \times K \times P \times K_B \times \sqrt{M}} $$

شیرهای تخلیه سیستم گاز و بخار (SCFM):

$$ A=\frac{S C F M \times \sqrt{T G Z}}{1.175 \times C \times K \times P \times K_B} $$

Definitions:

A: Minimum required effective relief valve discharge area (square inches)
GPM: Required relieving capacity at flow conditions (gallons per minute)
W: Required relieving capacity at flow conditions (pounds per hour)
SCFM: Required relieving capacity at flow conditions (standard cubic feet per minute)
G: Specific gravity of liquid, gas, or vapor at flow conditions (water = 1.0 for most HVAC applications; air = 1.0)
C: Coefficient determined from the expression of the ratio of specific heats (C = 315 if value is unknown)
K: Effective coefficient of discharge (K = 0.975)
KB: Capacity correction factor due to back pressure (KB = 1.0 for atmospheric discharge systems)
KV: Flow correction factor due to viscosity (KV = 0.9 to 1.0 for most HVAC applications with water)
KN: Capacity correction factor for dry saturated steam at set pressures above 1500 psia and up to 3200 psia (KN = 1.0 for most HVAC applications)
KSH: Capacity correction factor due to the degree of superheat (KSH = 1.0 for saturated steam)
Z: Compressibility factor (Z = 1.0 if value is unknown)
P: Relieving pressure (psia) (P = set pressure (psig) + overpressure (10% psig) + atmospheric pressure (14.7 psia))
∆P: Differential pressure (psig) (∆P = set pressure (psig) + overpressure (10% psig) − back pressure (psig))
T: Absolute temperature (°R = °F + 460)
M: Molecular weight of the gas or vapor

Relief Valve Sizing Notes:

هنگامی که از چندین شیر کمکی استفاده می شود، یک شیر باید در حداکثر فشار کاری مجاز یا کمتر از آن تنظیم شود و شیرهای باقیمانده ممکن است تا 5 درصد بیش از حداکثر فشار کاری مجاز تنظیم شوند.
هنگام اندازه گیری چندین شیر کمکی، کل مساحت مورد نیاز بر روی فشار بیش از حد 16 درصد یا 4 Psi محاسبه می شود که هر کدام بیشتر باشد.
برای بخار فوق گرم، می توان از مقادیر ضریب تصحیح فهرست شده در زیر استفاده کرد:

Superheat Calculator

Select Superheat (°F):

Selected Superheat: 0 °F

Correction Factor: 0.97

ارزش سوپرهیت	ضریب تصحیح
تا دمای 400 درجه فارنهایت گرم کنید	0.97 (محدوده 0.979-0.998)
تا دمای 450 درجه فارنهایت گرم کنید	0.95 (محدوده 0.957-0.977)
تا دمای 500 درجه فارنهایت گرم کنید	0.93 (محدوده 0.930-0.968)
سوپرگرم تا 550 درجه فارنهایت	0.90 (محدوده 0.905-0.974)
تا دمای 600 درجه فارنهایت سوپرگرم کنید	0.88 (محدوده 0.882-0.993)
تا دمای 650 درجه فارنهایت سوپرگرم کنید	0.86 (محدوده 0.861-0.988)
تا دمای 700 درجه فارنهایت سوپرگرم کنید	0.84 (محدوده 0.841-0.963)
تا دمای 750 درجه فارنهایت سوپرگرم کنید	0.82 (محدوده 0.823-0.903)
سوپرگرم تا 800 درجه فارنهایت	0.80 (محدوده 0.805-0.863)
سوپرگرم تا 850 درجه فارنهایت	0.78 (محدوده 0.786-0.836)
تا دمای 900 درجه فارنهایت گرم کنید	0.75 (محدوده 0.753-0.813)
تا دمای 950 درجه فارنهایت گرم کنید	0.72 (محدوده 0.726-0.792)
سوپرگرم تا 1000 درجه فارنهایت	0.70 (محدوده 0.704-0.774)

عوامل تصحیح طراحی شیر تسکین دهنده

Material Properties

Select Material:

Properties:

Molecular Weight:

Ratio of Specific Heats:

Coefficient C:

Specific Gravity:

You may use table instead of calculator

گاز یا بخار	وزن مولکولی	نسبت گرمای خاص	ضریب ج	گرانش مخصوص
استیلن	26.04	1.25	342	0.899
هوا	28.97	1.40	356	1.000
آمونیاک (R-717)	17.03	1.30	347	0.588
آرگون	39.94	1.66	377	1.379
بنزن	78.11	1.12	329	2.696
N-بوتان	58.12	1.18	335	2.006
ایزو بوتان	58.12	1.19	336	2.006
دی اکسید کربن	44.01	1.29	346	1.519
دی سولفید کربن	76.13	1.21	338	2.628
مونوکسید کربن	28.01	1.40	356	0.967
کلر	70.90	1.35	352	2.447
سیکلوهگزان	84.16	1.08	325	2.905
اتان	30.07	1.19	336	1.038
الکل اتیلیک	46.07	1.13	330	1.590
اتیل کلرید	64.52	1.19	336	2.227
اتیلن	28.03	1.24	341	0.968
هلیوم	4.02	1.66	377	0.139
N-هپتان	100.20	1.05	321	3.459
هگزان	86.17	1.06	322	2.974
اسید هیدروکلریک	36.47	1.41	357	1.259
هیدروژن	2.02	1.41	357	0.070
هیدروژن کلرید	36.47	1.41	357	1.259
سولفید هیدروژن	34.08	1.32	349	1.176
متان	16.04	1.31	348	0.554
متیل الکل	32.04	1.20	337	1.106
متیل بوتان	72.15	1.08	325	2.491
متیل کلراید	50.49	1.20	337	1.743
گاز طبیعی	19.00	1.27	344	0.656
اکسید نیتریک	30.00	1.40	356	1.036
نیتروژن	28.02	1.40	356	0.967
اکسید نیتروژن	44.02	1.31	348	1.520
اکتان N	114.22	1.05	321	3.943
اکسیژن	32.00	1.40	356	1.105
N-پنتان	72.15	1.08	325	2.491
ایزو پنتان	72.15	1.08	325	2.491
پروپان	44.09	1.13	330	1.522
R-11	137.37	1.14	331	4.742
R-12	120.92	1.14	331	4.174
R-22	86.48	1.18	335	2.985
R-114	170.93	1.09	326	5.900
R-123	152.93	1.10	327	5.279
R-134a	102.03	1.20	337	3.522
دی اکسید گوگرد	64.04	1.27	344	2.211
تولوئن	92.13	1.09	326	3.180

ویژگی های گاز و بخار برای محاسبه شیر تخلیه

FREQUENTLY ASKED QUESTIONS

What are the key factors that affect the performance of pressure relief valves?

The performance of pressure relief valves is affected by several key factors, including the valve’s set pressure, overpressure, and blowdown. The set pressure is the pressure at which the valve opens, while overpressure is the amount by which the system pressure exceeds the set pressure. Blowdown, on the other hand, is the difference between the set pressure and the pressure at which the valve reseats. Other factors that can impact performance include the valve’s flow coefficient, valve size, and the properties of the fluid being relieved.

How do I determine the required relief valve orifice area?

The required relief valve orifice area can be determined using the API 520/521 equations, which take into account the valve’s flow coefficient, the relieving pressure, and the required flow rate. The orifice area is typically calculated using the following equation: A = Q / (CKP), where A is the orifice area, Q is the required flow rate, C is the flow coefficient, K is the valve’s discharge coefficient, and P is the relieving pressure.

What is the significance of the relief valve vent line maximum length?

The relief valve vent line maximum length is critical because it affects the valve’s ability to relieve pressure safely and efficiently. A vent line that is too long can lead to excessive backpressure, which can prevent the valve from opening fully or cause it to reseat prematurely. The maximum length of the vent line can be calculated using the equation provided in the API 520/521 standards, which takes into account the valve’s set pressure, the vent line’s diameter, and the density of the fluid being relieved.

How do I select the correct relief valve for my application?

Selecting the correct relief valve for your application involves considering several factors, including the system’s design pressure, the relieving pressure, and the required flow rate. You should also consider the type of fluid being relieved, as well as any specific regulatory requirements or industry standards that apply. Other factors to consider include the valve’s material construction, its flow characteristic, and its certification or approval by relevant authorities.

What are the different types of pressure relief valves available?

There are several types of pressure relief valves available, including spring-loaded valves, pilot-operated valves, and rupture discs. Spring-loaded valves are the most common type and are suitable for most applications. Pilot-operated valves, on the other hand, are typically used for high-flow applications or where a high degree of accuracy is required. Rupture discs are used in applications where a rapid release of pressure is required, such as in fire suppression systems.

How do I ensure the proper installation and maintenance of pressure relief valves?

Proper installation and maintenance of pressure relief valves are critical to ensure their safe and efficient operation. Installation should be carried out in accordance with the manufacturer’s instructions and relevant industry standards. Regular maintenance should include inspections, testing, and cleaning of the valve to ensure it remains functional and free from blockages or corrosion.

What are the consequences of inadequate pressure relief valve sizing?

Inadequate pressure relief valve sizing can have serious consequences, including equipment damage, injury to personnel, and environmental harm. Undersized valves may not be able to relieve pressure quickly enough, leading to a buildup of pressure that can cause catastrophic failures. Oversized valves, on the other hand, can lead to excessive flow rates and energy losses. Proper sizing of pressure relief valves is therefore critical to ensure safe and efficient operation of process equipment.